�A 1.55 �m hybrid InGaAsP-Si laser was fabricated by the

selective area metal bonding method. Room temperature continuous lasing with a maximum output power of 0.45 mW is realized.

Index Terms—Selective area metal bonding (SAMB), InGaAsP-Si Laser, Si photonics

I. INTRODUCTION ECENTLY, ever increasing attention has been paid to the Si photonics [1], [2]. Though the passive devices of Si photonics are well developed [3]–[6], the Si light sources especially Si lasers are still a crucial obstacle in Si photonics [7], [8]. Many efforts have been paid in

this field [9]–[11]. Fang et al have proposed the plasma-assisted low temperature direct bonding method [12] and fabricated electrically pumped hybrid Si based evanescent lasers successfully [13]–[15]. Very recently, we developed a selective area metal bonding (SAMB) method [16] which laterally separates the optical coupling area and the metal bonding area to avoid strong light absorption of metal in optical coupling area when a Si waveguide is integrated with an InGaAsP structure. A room-temperature continuous-wave electrically pumped InGaAsP-Si hybrid laser has been fabricated by the SAMB method. The SAMB method inherits the following advantages of metal bonding: low thermal stress, no critical cleanness requirement, superiorly electrical performance [17]–[19]. Different types of laser dies with various sizes may be bonded onto any desired places of a Si on Insulator (SOI) substrate while other parts of the substrate could be left untouched for the fabrication of Si passive devices, which provides flexibility for the application of the SAMB method in Si photonics. The SAMB method may be extended into the fabrication of other Si hybrid photonic devices.

This work was supported by the National Natural Science Foundation of

China (No. 10874001; 50732001; 10674012; 60877022) and National Basic Research Program of China (973 Program, No. 2007CB613402).

T. Hong, S. Yue, W. X. Chen, Z. Li and G. Z. Ran are with the State Key Lab for Mesoscopic Physics and School of Physics, Peking University, Beijing 100871, China (corresponding author, phone:86-10-62751618; fax: 86-10-62751615;e-mail: rangz@pku.edu.cn)

Y. Wang, H. Y. Yu, S. Liang, and J. Q. Pan are with Key Lab of Semiconductor Materials Science, Institute of Semiconductors, CAS, Beijing Beijing 100083, China.

II. DEVICE STRUCTURE AND FABRICATION The schematic diagram of the hybrid InGaAsP-Si laser

fabricated by the SAMB method is shown in Fig. 1. A thinned InGaAsP multiple quantum well (MQW) distributed feedback (DFB) waveguide laser with a 3 �m ridge width and 680 �m cavity length is bonded upon to a pre-patterned SOI wafer in the selective area by bonding metals. The detailed parameters of the InGaAsP MQW DFB waveguide laser can be seen in [16]. The 100 nm InGaAsP etch-stop layer below the MQW layers in our structure is used to thin the InGaAsP structure flatly and smoothly to the thickness of about 3 �m (from the top of ridge to the bottom InGaAsP structure), which is a key point to avoid cracking of the thinned InGaAsP structure during bonding and to decrease the evanescent wave scattering loss in the optical coupling area.

In the SOI part, two Si blocking stripes on the two sides of the central Si waveguide are introduced to prevent the metals from flowing to the central Si waveguide during bonding, thus avoiding the strong light absorption of metals in the optical coupling area. Both the Si waveguide and the Si blocking stripes have the height of 800 nm and their widths are 5 and 2 �m respectively.

Fig. 1. 3D illustration of a hybrid InGaAsP-Si laser based on the SAMB

method. An air gap may exist between the InGaAsP structure and Si

waveguide if the height of the bonding metals is slightly higher than that of the Si waveguide because of the tolerance of evaporation thickness. In order to reduce the air gap width, a 4 �m excess metals accommodated space beside each blocking stripe is designed. The excess metals, if any, can flow to the spaces during bonding and reduce the total height of bonding metals to that of Si waveguide, shrinking the air gap.

The top view micrographs of the main steps for fabricating an InGaAsP-Si hybrid laser by the SAMB method are shown in

A Selective Area Metal Bonding Method for Si Photonics Light Sources

Tao Hong, Yang Wang, Hong-Yan Yu, Song Yue, Wei-Xi Chen, Song Liang, Zhi Li, Jiao-Qing Pan, Guang-Zhao Ran

R

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WB3 11:00 – 11:15

978-1-4244-6346-6/10/$26.00 ©2010 IEEE

Fig .2. The InGaAsP structure was epitaxial grown by MOCVD. The DFB structure of the InGaAsP laser is fabricated in the upper separate confinement heterostructure layer by conventional holographic exposure combined with standard photolithography process. The thin n-InP layer upon the DFB layer induces weak gain coupling into the DFB structure. The InGaAsP waveguide laser was adhered to a glass chip by wax and then the InP substrate was etched by HCl solution. The etching process stopped automatically at the etch-stop layer and smooth InGaAsP surface was present. Fig .2(a) is a top view of the thinned InGaAsP structure. On the other hand, the cleaned SOI wafer with 800 nm top Si and 3 �m SiO2 was selective etched by inductively coupled plasma (ICP) dry etch, and three Si stripes were left. The bonding metals were deposited onto the bonding areas of the SOI wafer by thermal evaporation. A micrograph of the SOI with the bonding metals is shown in Fig .2(b). The thinned InGaAsP structure and Si waveguide were aligned and bonded together by a bonder with a sub-micrometer precision. The bonding process is done at 300

in N2 atmosphere for 10 min under a pressure of about 2 MPa. Finally, the glass chip was removed by dipping the device in acetone for several minutes. Fig .2(c) is a micrograph of a well aligned and bonded device.

Fig .2 The top view micrographs of the main process steps of fabricating an InGaAsP-Si hybrid laser. (a) A thinned InGaAsP structure; (b) A patterned SOI wafer with bonding metals; (c) A well aligned and bonded device.

The bonding metals on the SOI wafer not only function as

a bonding media, but also provide an Ohmic contact to the InGaAsP structure. Metal evaporation sequence is AuGeNi(100 nm)/In(680 nm)/Sn(20 nm). AuGeNi is used to form a good Ohmic contact to n-type InGaAsP and a good adhesiveness with the SiO2 layer of the SOI wafer. Indium is chosen as the main bonding medium due to its low melting point, good fluidness and low stress accumulation. A thin Sn film above the In layer is deposited to prevent In from oxidation.

III. EXPERIMENTAL RESULTS Figure 3(a) shows the SEM image of the cross section of a

typical hybrid InGaAsP-Si laser made by the SAMB method. As shown, the III-V ridge structure and Si waveguide are aligned well and touched tightly. There is no observable air gap between the InGaAsP structure and Si waveguide. If any, it should be under the resolution of SEM (about 10 nm). The gap may increase slightly due to thermal stress under driving current. Fig. 3(b) is a near-field image taken from an unpolished end facet of the hybrid laser which was focused at the end facet of Si waveguide. The laser spot whose width is approximately equal to that of the Si waveguide covers the Si waveguide region. In addition, we have made a passive theoretical simulation based on the structure of our device. The simulation results show that the light generated in the InGaAsP waveguide laser can be coupled into the Si waveguide effectively. Fig. 3(c) shows the simulation results of the steady optical field after a certain distance of light propagation, which agrees with Fig. 3(b).

(a)

(b) (c)

Fig. 3 (a) SEM image of the cross section of the hybrid InGaAsP-Si laser. (b) A near-field image taken from an end facet of the hybrid InGaAsP-Si laser. (c) The steady optical field after a certain distance of light propagation based on passive simulation.

The p contact of the hybrid laser is the top TiAu layer of the InGaAsP ridge laser while the n contact is formed by the bonding metals on the SOI wafer. The total laser output power

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confined both in Si waveguide and III-V layers is collected from one end facet of the laser. The emission spectrum of the hybrid laser is shown in the inset of Fig. 4. A single longitudinal mode with a wavelength of 1554 nm is presented, which indicates that the DFB structure has not been destroyed during etching and bonding. Fig. 4 shows the typical curves of light output power versus continuous-wave injection current at various temperatures. The maximum output power is 0.45 mW at the temperature of 10 . A kink turning up in each curve in Fig. 4 indicates that the laser probably operates in two modes at large injection current, which is due to no 1/4 � phase shift in our DFB structure. The output power decreases with injection current after passing through a maximum value. The possible reason is that the series resistance of the hybrid laser is not optimized. The roughly estimated thermal impedance is evidently larger than that of a common DFB laser.

Fig. 4. Typical curves of light output power versus continuous-wave injection current at various temperatures. Inset is the emission spectrum.

The major reasons for the relative low output of our present device under continuous-wave condition are as follows: the series resistance of our present laser is large and the Si waveguide etched in our laboratory is not very smooth, causing a large loss. Several strategies will be applied to promote the light output power in future work, such as improving our etching technology to obtain perfect Si waveguide and optimizing the bonding metals and bonding conditions.

IV. CONCLUSION

By using the SAMB method, a 1.55 �m hybrid InGaAsP-Si laser operating with a maximum output power of 0.45 mW under continuous-wave condition has been realized. The advantages of simplicity and flexibility of the fabrication process enable the SAMB method to be a promising approach to fabricate effective Si light sources for Si photonics and to make other Si hybrid photonic devices.

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