- Email: [email protected]
Wafer direct bonding of compound semiconductors and silicon at room temperature by the surface activated bonding method
applied
surface science ELSEVIER
Applied Surface Science 117/l
18 (1997) 808-812
Wafer direct bonding of compound semiconductors and silicon at room temperature by the surface activated bonding method Taek Ryong Chung a,*, Liu Yang b, Naoe Hosoda a, Hidegi Takagi ‘, Tadatomo Suga a a RCAST, The University of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 153, Japan b Department of Materials Engineering, Southwest Jiotong University, Chengdu 610031, China ’ Mechanical Engineering Laboratory AIST, MITI, Namiki 1-2, Tsukuba, Ibaraki 305, Japan
Abstract The surface activated bonding method has been applied to bond the III-V compound semiconductor wafers and Si wafer directly at room temperature in ultra high vacuum. The procedure is as follows: the surfaces to be bonded are sputter-cleaned and activated by Ar fast atom beam irradiation and brought into contact under slight pressure. The GaAs and Si wafers bonded very well, without any macro defect being detected along the bonded interface. An amorphous intermediate layer of about 3 nm thick in most parts of the interface boundary direct bonding without any interlayer between the GaAs and Si wafers existed in a partially bonded interface by high resolution TEM. Keywords: SAB; Compound
semiconductor;
Silicon; Interface
1. Introduction The fabrication of dissimilar semiconductor materials on Si would facilitate new types of integrated devices, such as photonic integrated circuits and optoelectronic integrated circuits. The key technology to fabricate it has so far been heteroepitaxial growth. However, this technology for integrating different materials suffers from several significant disadvantages, such as a large lattice mismatch, which causes a high density of threading dislocation and a large difference in thermal expansion coeffi-
* Corresponding author. Tel.: +81-3-3481-4490; 34814489; e-mail: [email protected]. 0169-4332/97/$17.00 Copyright PII SOl69-4332(97)00147-5
fax: +81-3-
cients, which leads to large residual thermal stress. Another approach to realize the integration is a wafer direct bonding technique, because this technique can be used to fabricate highly lattice-mismatched heterostructures without any conventional threading dislocation [l-3], for example, fabrication of (OOl)InPbased 1.55 mm wavelength lasers on a (110)GaAs substrate by direct bonding [4]. High-quality InGaAs/InP MQW structures were fabricated on Si substrates by direct bonding at 700°C [5]. The fusion of compound semiconductor crystals on a macroscopic scale has been demonstrated and has produced pn junctions with normal diode characteristics [6]. The conventional bonding method may lead to various problems such as doping of impurity, thermal stress introduction, defect generation and metal
0 1997 Elsevier Science B.V. All rights reserved.
T.R. Chung et al/Applied
Surface Science 117/118
wiring corruption because of the high temperature processing. Thus, low temperature bonding technology has become important in realizing highly integrated micro electro and mechanical devices. A new technology for fabricating III-VI optical devices on Si, the surface activated bonding (SAB) [7-101 method, has been developed at room temperature. The surface activated bonding method realizes room temperature bonding in a vacuum. The primary concept of the method is based on the idea that clean surfaces are inherently active and react with other elements such as oxygen, nitrogen and carbon even at room temperature. In this procedure, the surfaces are bonded by an argon fast atom beam and then brought into contact with each other in an ultrahigh vacuum. We have investigated this bonding between metals and ceramics [8]. In this letter, we report a surface activated bonding method to fabricate III-VI optical devices on Si. Using the SAB method, compound semiconductors and Si(100) wafers were successfully bonded at room temperature.
2. Experiments In the bonding experiment, we used the 10 X 10 mm specimens cut from the Si(lOO)-350 pm,
Preparation chamber
(1997) 808412
809
GaAs(lOO)-560 pm and InP(lOO)-350 pm wafers by a diamond saw. The InP wafer was doped with Fe. These specimens were first washed in acetone, ethanol and deionized water to remove the photoresist and wax used for cutting. The Si wafers were dipped in H, SO, : H,O, = 4 : 1 mixture at 80°C for 10 min. The GaAs and InP wafers were dipped in acetone and ethanol. Fig. 1 is a schematic diagram of the surface activated bonding apparatus. The apparatus consists of an ultra high vacuum chamber, an Auger analysis chamber, a transfer and a preparation chamber with a fast atom beam (FAB) source. The mechanically and chemically polished surfaces of the specimens were activated by sputter etching with an argon atom beam. Etching conditions were 1.5 kV applied voltage and 15 mA plasma current for each gun. The surfaces were activated by means of argon fast atom beam bombardment for 300 s in a vacuum of 10e6 Pa. The beam incident angle was 45”. After the etching, two specimens were brought into contact with a pushing pressure of 20 MPa in a vacuum of lo-’ Pa. The time interval between the end of FAB and the start of bonding was about 20 min. To analysis the surface roughness of the specimen by an atomic force microscope @FM) and the surface properties of the specimen by the Auger electron spectra (AES), the irradiation time was varied from
Transfer chamber
Preparation chamber
Pimp
AES
FAB Bonding chamber
Pump
Ar beam source
Analysis chamber
Fig. 1. Schematic diagram of the surface activated bonding apparatus.
810
T.R. Chung et al./Applied
Surface Science 117/118
300 to 1800 s. To check voids at the bonded interface we used an acoustic microscope system of OLYMPUS UH, pulse 200. To prepare the thin foils for TEM observation, bonded joints consolidated by epoxy resin were cut into 1 mm sheets by a low speed diamond saw and mechanically polished with No. 1500 grinding paper to about 500 pm, followed by dimple polishing to about 50 pm. Final thinning was conducted by ion-beam thinning in a Gatan-600 ion milling machine with a 4 kV gun voltage and ca. 1.0 mA gun current.
3. Results Fig. 2 shows the results of the Auger electron spectroscopy. Before irradiation, the surface of the
600
(1997) 808-812
specimen consisted of oxygen and a small proportion of carbon. These surface contaminants decreased drastically after 69 s irradiation. After 60 s irradiation, the spectrum reached a constant state and did not change until after 900 s irradiation. Fig. 3a) shows the AFM profile of the GaAs wafer surface before Ar beam irradiation and after 900 s irradiation. The surface roughness decreased from 0.643 to 0.441 nm Rms. Fig. 3b) shows the AFM profile of the InP wafer surface before Ar beam irradiation and after 300 s irradiation. The surface roughness increased from 1.229 to 5.295 nm Rms. Fig. 4 shows the acoustic microscope topography of the Si-GaAs wafer bonded at room temperature by the surface activated bonding method. The Si wafer and GaAs wafer were joined well. No micro-defects were ob-
1200
Kinetic Energy(eV)
Kinetic Energy(eV)
AP FAB(Fd Atom Beam) Accelerate Voltage : 1.6 IN,
16mA
Fig. 2. Auger electron spectra of (a) GaAs and (b) InP specimens along with Ar‘fast atom beam irradiation.
811
T.R. Chung et al. /Applied Surface Science I1 7/ I 18 (1997) 808-812
Fig. 3. (a) AFM image of the GaAs specimen before Ar beam irradiation specimen before Ar beam irradiation and after 300 and 1800 s irradiation.
served by the acoustic microscope. The average tensile strength of the joins is measured as being about 10 MPa. HREM study of the bonded interfaces confirmed that there are probably amorphous layers on both. Fig. 5 shows the TEM image of the interface boundaries in the bonded GaAs and Si wafers by the surface activated bonding method. Bonding is satisfactory in that no crack or pore was observed after cutting and after mechanical grinding perpendicular to the bonding interface. In most parts of the interface, general viewing of the interface boundary indicated that an intermediate layer about 3 nm thick existed along the interface. However, in some parts, the GaAs and Si wafers have been bonded directly. In the case of the bonding of InP and Si wafers, an intermediate layer of 200 nm, which was determined as being amorphous by selected area electron diffraction, was observed between the InP (doped layer) and Si. The amorphous layer observed seemed to grow from the silicon, judging by the morphology of the Si and amorphous interface. This amorphous layer could have been formed during the 20 min interval between the SAB and bonding, due to the residual moisture environment. There seemed to be a certain kind of precipitate, 30-50 nm large, embed-
and after 300 and 900 s irradiation.
(b) AFM image of the InP
ded in the InP matrix. However, according to the HRTEM photos, the lattice image was continuous between the matrix and precipitates, although there was a sharp contrast between them. The particles were too small for selected area diffraction. However, in some areas, direct bonding was identified by a high resolution lattice image of the interface. Ac-
Fig. 4. Acoustic microscope topography GaAs/Si wafer at room temperature.
of SAB method bonded
812
T.R. Chung et al./Applied
Surface Science 117/ 118 (1997) 808-812
Fig. 5. High resolution TEM image of the interface boundary
cording to EDX/TEM analysis, the intermediate layers were mainly composed of silicon.
4. Conclusion In conclusion, using the surface activated bonding method, GaAs(lOO)-Si(lOO), InP(lOO)-Si(l00) wafers were successfully bonded at room temperature. The surface activated bonding method can be applied to fabricate III-VI optical devices on Si. High resolution TEM has been used to observe the GaAs and Si wafers interface microstructure in the bonded joints. An amorphous intermediate layer of thickness less than 2-3 nm was found at the bonded interfaces of GaAs and Si wafers. Direct bonding existed in the paryial areas of the interface by high resolution TEM. The intermediate layer is mainly composed of silicon.
References [I] Y.H. Lo, R. Bhat, D.M. Hwang, M.A. Koza, T.P. Lee, Appl. Phys. Lett. 58 (1991) 1961.
in as-bonded
GaAs/Si
wafer.
[2] H. Wada, Y. Ogawa, T. Kamijoh, Appl. Phys. Lett. 62 (1993) 738. [3] J.J. Dudley, D.I. Babic, R. Mirin, L. Yang, B.I. Miller, R.J. Ram, T. Reynolds, E.-L. Hu, J.E. Bowers, Appl. Phys. Lett. 64 (1994) 1463. [4] Y. Okuno, M. Aoki, T. Tsuchiya, K. Uomi, Appl. Phys. Lett. 67 (1995) 810. [5] K. Mori, K. Tokutume, K. Nishi, S. Sugou, Electron. Lett. 30 (12) (1994) 1008. [6] Z.L. Liau, D.E. Mull, Appl. Phys. Lett. 56 (1990) 737. [7] T. Suga, K. Miyazawa, Acta Ser. Metall. Proc. Ser. 4 (1990) 189. [8] T. Suga, Y. Takahashi, H. Takagi, B. Gibbesch, G. Elssner, Acta Metall. Mater. 40 (1992) S113. [9] T. Suga, Y. Takahashi, H. Takagi, Ceram. Trans. 35 (1993) 323. [lo] T.R. Chung, L. Yang, N. Hosoda, T.Suga, Nucl. Instr. Meth. B 121 (1997) 203.
surface science ELSEVIER
Applied Surface Science 117/l
18 (1997) 808-812
Wafer direct bonding of compound semiconductors and silicon at room temperature by the surface activated bonding method Taek Ryong Chung a,*, Liu Yang b, Naoe Hosoda a, Hidegi Takagi ‘, Tadatomo Suga a a RCAST, The University of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 153, Japan b Department of Materials Engineering, Southwest Jiotong University, Chengdu 610031, China ’ Mechanical Engineering Laboratory AIST, MITI, Namiki 1-2, Tsukuba, Ibaraki 305, Japan
Abstract The surface activated bonding method has been applied to bond the III-V compound semiconductor wafers and Si wafer directly at room temperature in ultra high vacuum. The procedure is as follows: the surfaces to be bonded are sputter-cleaned and activated by Ar fast atom beam irradiation and brought into contact under slight pressure. The GaAs and Si wafers bonded very well, without any macro defect being detected along the bonded interface. An amorphous intermediate layer of about 3 nm thick in most parts of the interface boundary direct bonding without any interlayer between the GaAs and Si wafers existed in a partially bonded interface by high resolution TEM. Keywords: SAB; Compound
semiconductor;
Silicon; Interface
1. Introduction The fabrication of dissimilar semiconductor materials on Si would facilitate new types of integrated devices, such as photonic integrated circuits and optoelectronic integrated circuits. The key technology to fabricate it has so far been heteroepitaxial growth. However, this technology for integrating different materials suffers from several significant disadvantages, such as a large lattice mismatch, which causes a high density of threading dislocation and a large difference in thermal expansion coeffi-
* Corresponding author. Tel.: +81-3-3481-4490; 34814489; e-mail: [email protected]. 0169-4332/97/$17.00 Copyright PII SOl69-4332(97)00147-5
fax: +81-3-
cients, which leads to large residual thermal stress. Another approach to realize the integration is a wafer direct bonding technique, because this technique can be used to fabricate highly lattice-mismatched heterostructures without any conventional threading dislocation [l-3], for example, fabrication of (OOl)InPbased 1.55 mm wavelength lasers on a (110)GaAs substrate by direct bonding [4]. High-quality InGaAs/InP MQW structures were fabricated on Si substrates by direct bonding at 700°C [5]. The fusion of compound semiconductor crystals on a macroscopic scale has been demonstrated and has produced pn junctions with normal diode characteristics [6]. The conventional bonding method may lead to various problems such as doping of impurity, thermal stress introduction, defect generation and metal
0 1997 Elsevier Science B.V. All rights reserved.
T.R. Chung et al/Applied
Surface Science 117/118
wiring corruption because of the high temperature processing. Thus, low temperature bonding technology has become important in realizing highly integrated micro electro and mechanical devices. A new technology for fabricating III-VI optical devices on Si, the surface activated bonding (SAB) [7-101 method, has been developed at room temperature. The surface activated bonding method realizes room temperature bonding in a vacuum. The primary concept of the method is based on the idea that clean surfaces are inherently active and react with other elements such as oxygen, nitrogen and carbon even at room temperature. In this procedure, the surfaces are bonded by an argon fast atom beam and then brought into contact with each other in an ultrahigh vacuum. We have investigated this bonding between metals and ceramics [8]. In this letter, we report a surface activated bonding method to fabricate III-VI optical devices on Si. Using the SAB method, compound semiconductors and Si(100) wafers were successfully bonded at room temperature.
2. Experiments In the bonding experiment, we used the 10 X 10 mm specimens cut from the Si(lOO)-350 pm,
Preparation chamber
(1997) 808412
809
GaAs(lOO)-560 pm and InP(lOO)-350 pm wafers by a diamond saw. The InP wafer was doped with Fe. These specimens were first washed in acetone, ethanol and deionized water to remove the photoresist and wax used for cutting. The Si wafers were dipped in H, SO, : H,O, = 4 : 1 mixture at 80°C for 10 min. The GaAs and InP wafers were dipped in acetone and ethanol. Fig. 1 is a schematic diagram of the surface activated bonding apparatus. The apparatus consists of an ultra high vacuum chamber, an Auger analysis chamber, a transfer and a preparation chamber with a fast atom beam (FAB) source. The mechanically and chemically polished surfaces of the specimens were activated by sputter etching with an argon atom beam. Etching conditions were 1.5 kV applied voltage and 15 mA plasma current for each gun. The surfaces were activated by means of argon fast atom beam bombardment for 300 s in a vacuum of 10e6 Pa. The beam incident angle was 45”. After the etching, two specimens were brought into contact with a pushing pressure of 20 MPa in a vacuum of lo-’ Pa. The time interval between the end of FAB and the start of bonding was about 20 min. To analysis the surface roughness of the specimen by an atomic force microscope @FM) and the surface properties of the specimen by the Auger electron spectra (AES), the irradiation time was varied from
Transfer chamber
Preparation chamber
Pimp
AES
FAB Bonding chamber
Pump
Ar beam source
Analysis chamber
Fig. 1. Schematic diagram of the surface activated bonding apparatus.
810
T.R. Chung et al./Applied
Surface Science 117/118
300 to 1800 s. To check voids at the bonded interface we used an acoustic microscope system of OLYMPUS UH, pulse 200. To prepare the thin foils for TEM observation, bonded joints consolidated by epoxy resin were cut into 1 mm sheets by a low speed diamond saw and mechanically polished with No. 1500 grinding paper to about 500 pm, followed by dimple polishing to about 50 pm. Final thinning was conducted by ion-beam thinning in a Gatan-600 ion milling machine with a 4 kV gun voltage and ca. 1.0 mA gun current.
3. Results Fig. 2 shows the results of the Auger electron spectroscopy. Before irradiation, the surface of the
600
(1997) 808-812
specimen consisted of oxygen and a small proportion of carbon. These surface contaminants decreased drastically after 69 s irradiation. After 60 s irradiation, the spectrum reached a constant state and did not change until after 900 s irradiation. Fig. 3a) shows the AFM profile of the GaAs wafer surface before Ar beam irradiation and after 900 s irradiation. The surface roughness decreased from 0.643 to 0.441 nm Rms. Fig. 3b) shows the AFM profile of the InP wafer surface before Ar beam irradiation and after 300 s irradiation. The surface roughness increased from 1.229 to 5.295 nm Rms. Fig. 4 shows the acoustic microscope topography of the Si-GaAs wafer bonded at room temperature by the surface activated bonding method. The Si wafer and GaAs wafer were joined well. No micro-defects were ob-
1200
Kinetic Energy(eV)
Kinetic Energy(eV)
AP FAB(Fd Atom Beam) Accelerate Voltage : 1.6 IN,
16mA
Fig. 2. Auger electron spectra of (a) GaAs and (b) InP specimens along with Ar‘fast atom beam irradiation.
811
T.R. Chung et al. /Applied Surface Science I1 7/ I 18 (1997) 808-812
Fig. 3. (a) AFM image of the GaAs specimen before Ar beam irradiation specimen before Ar beam irradiation and after 300 and 1800 s irradiation.
served by the acoustic microscope. The average tensile strength of the joins is measured as being about 10 MPa. HREM study of the bonded interfaces confirmed that there are probably amorphous layers on both. Fig. 5 shows the TEM image of the interface boundaries in the bonded GaAs and Si wafers by the surface activated bonding method. Bonding is satisfactory in that no crack or pore was observed after cutting and after mechanical grinding perpendicular to the bonding interface. In most parts of the interface, general viewing of the interface boundary indicated that an intermediate layer about 3 nm thick existed along the interface. However, in some parts, the GaAs and Si wafers have been bonded directly. In the case of the bonding of InP and Si wafers, an intermediate layer of 200 nm, which was determined as being amorphous by selected area electron diffraction, was observed between the InP (doped layer) and Si. The amorphous layer observed seemed to grow from the silicon, judging by the morphology of the Si and amorphous interface. This amorphous layer could have been formed during the 20 min interval between the SAB and bonding, due to the residual moisture environment. There seemed to be a certain kind of precipitate, 30-50 nm large, embed-
and after 300 and 900 s irradiation.
(b) AFM image of the InP
ded in the InP matrix. However, according to the HRTEM photos, the lattice image was continuous between the matrix and precipitates, although there was a sharp contrast between them. The particles were too small for selected area diffraction. However, in some areas, direct bonding was identified by a high resolution lattice image of the interface. Ac-
Fig. 4. Acoustic microscope topography GaAs/Si wafer at room temperature.
of SAB method bonded
812
T.R. Chung et al./Applied
Surface Science 117/ 118 (1997) 808-812
Fig. 5. High resolution TEM image of the interface boundary
cording to EDX/TEM analysis, the intermediate layers were mainly composed of silicon.
4. Conclusion In conclusion, using the surface activated bonding method, GaAs(lOO)-Si(lOO), InP(lOO)-Si(l00) wafers were successfully bonded at room temperature. The surface activated bonding method can be applied to fabricate III-VI optical devices on Si. High resolution TEM has been used to observe the GaAs and Si wafers interface microstructure in the bonded joints. An amorphous intermediate layer of thickness less than 2-3 nm was found at the bonded interfaces of GaAs and Si wafers. Direct bonding existed in the paryial areas of the interface by high resolution TEM. The intermediate layer is mainly composed of silicon.
References [I] Y.H. Lo, R. Bhat, D.M. Hwang, M.A. Koza, T.P. Lee, Appl. Phys. Lett. 58 (1991) 1961.
in as-bonded
GaAs/Si
wafer.
[2] H. Wada, Y. Ogawa, T. Kamijoh, Appl. Phys. Lett. 62 (1993) 738. [3] J.J. Dudley, D.I. Babic, R. Mirin, L. Yang, B.I. Miller, R.J. Ram, T. Reynolds, E.-L. Hu, J.E. Bowers, Appl. Phys. Lett. 64 (1994) 1463. [4] Y. Okuno, M. Aoki, T. Tsuchiya, K. Uomi, Appl. Phys. Lett. 67 (1995) 810. [5] K. Mori, K. Tokutume, K. Nishi, S. Sugou, Electron. Lett. 30 (12) (1994) 1008. [6] Z.L. Liau, D.E. Mull, Appl. Phys. Lett. 56 (1990) 737. [7] T. Suga, K. Miyazawa, Acta Ser. Metall. Proc. Ser. 4 (1990) 189. [8] T. Suga, Y. Takahashi, H. Takagi, B. Gibbesch, G. Elssner, Acta Metall. Mater. 40 (1992) S113. [9] T. Suga, Y. Takahashi, H. Takagi, Ceram. Trans. 35 (1993) 323. [lo] T.R. Chung, L. Yang, N. Hosoda, T.Suga, Nucl. Instr. Meth. B 121 (1997) 203.