KR100612048B1 - Chip bonding method - Google Patents

Abstract

The present invention relates to flip chip bonding for coupling between an optical active device chip such as a laser diode and a photo diode and an optical waveguide platform. In particular, a solder is formed only on a plating layer of the chip, and the platform affects the composition of the solder. Bonding multiple chips on a single platform by forming an UBM (Under Bump Metal) film for solder pads of less than 1 μm, and bonding them together by heating them to temperatures above the melting point and optical waveguide platforms below the melting point. This improves the problem of limited number of bonding due to oxidation of the solder. In addition, by incorporating a relatively expensive solder process in the chip manufacturing process with a high mass production rate compared to the method of forming a solder on a conventional platform, it is possible to expect the cost of the entire optical module.

Hybrid Integrated Optical Module, Silica PLC Platform, Multichip Bonding, Multiple Flipchip Bonding

Description

Chip Bonding Method

1A and 1B are cross-sectional views showing the position of solder in a flip chip bonding method according to the prior art.

Figure 2 is a process cross-sectional view showing a process of a multiple flip chip bonding method according to the prior art.

Figure 3 is a cross-sectional view showing the position of the solder for the multiple flip chip bonding method according to the present invention.

4 is a cross-sectional view illustrating a process of a multiple flip chip bonding method according to an embodiment of the present invention.

<Explanation of symbols for the main parts of the drawings>

31 optical active element chip 32 plating layer

33: solder 35: optical waveguide platform

36: UBM membrane

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flip chip bonding method for coupling between a plurality of optical active device chips and an optical waveguide platform, and more particularly, to a bonding method for maintaining a considerable degree of alignment between optical active devices so as not to reduce optical waveguide efficiency. .

Wavelength Division Multiplexing (WDM), which extends the transmission capacity by loading multiple wavelengths in one optical fiber, has been successfully introduced in the inter-station network, which solves traffic bottlenecks in subscriber networks and provides various service quality and high-speed data. As an alternative for transmission, a Wavelength Division Multiplexing-Passive Optical Network (WDM-PON) that can allocate independent wavelengths per subscriber has attracted attention. A key issue in the development of such WDM-PON is that the optical modules that make up the network must be supplied at low cost as it is utilized in subscriber networks along with high-speed signal transmission and wavelength stability. Therefore, the hybrid integrated optical module based on the optical waveguide platform that can integrate the light source / photodetector and the optical multiplexer / demultiplexer on one substrate has the advantages of excellent mass production and low cost of packaging. There is active research to preoccupy the network initial market and service.

Element technologies necessary for the fabrication of hybrid integrated optical modules include optical waveguide manufacturing technology, platform formation technology through post-processing such as depth etching and metal deposition, and flip chip bonding for mounting optical active device chips on the fabricated optical waveguide platform. Technology. The optical module constituting the WDM-PON is also manufactured with the above technique, especially in the case of an increase in the number of optical active elements mounted on the optical waveguide platform in proportion to an increase in the number of subscribers (or channels). The flip chip bonding in the held state becomes more difficult.

Referring to FIGS. 1A, 1B, and 2, a solder formation and a multiple flip chip method for flip chip bonding according to the prior art will be described.

Referring to FIG. 1A, an under bump metal (UBM) film 16 for wire and solder pad functions corresponding to an electrode 12 of an optical active device chip 11 on an optical waveguide platform 15 formed on a silicon substrate. ). The UBM film 16 may be formed by, for example, a method of sequentially thermally depositing or electron beam-depositing a metal such as Ti / Pt / Au or Cr / Ni / Au, and lifting-off or the like. The pattern is formed using the method of. Thereafter, a solder 17 made of an alloy such as InPb, PbSn or AuSn is deposited on the UBM film 16.

Referring to FIG. 1B, although not shown on the photoactive device chip 11 ′, a seed layer is formed by depositing a predetermined metal, and then a photoresist is applied to a surface to be plated. A photolithography process is performed to form a plating layer pattern, and then a plating layer 12 ′ is formed along the formed pattern. Gold (Au) may be used as a material forming the plating layer 12 ′. Solder 13 'is formed on the formed plating layer 12', and an alloy material is deposited by an E-beam evaporator. In this case, a ternary alloy composed of platinum (Pt), tin (Sn), and gold (Au) is used as the alloy material. Meanwhile, in the optical waveguide platform 15 ', the solder 17' is formed by following the process of FIG. 1A, and the material of the solder 17 'is 7: 3 by weight of gold (Au) and tin (Sn). The alloy mixed with can be used.

Referring to FIG. 2, a method of joining a plurality of optical active device chips 11 on one optical waveguide platform 15 by heat treating the solder 17 formed on the platform 15 is performed. Conventional flip chip bonding is to heat the platform 15 above the temperature at which the solder 17 melts (hereinafter melting point) to bond the individual chips 11 and the platform 15 in a thermocompression bonding manner.

In forming the solder according to the prior art as described above, all or part of the composition of the solder must be deposited on the optical waveguide platform. Typically, the flip chip bonding method using the above structure heats the platform above the melting point of the solder to melt the solder, and loads the chip so that the chip and the platform are joined by a thermocompression bonding method.

However, the flip chip bonding method has a disadvantage in that, as the number of bonding increases, the bonding strength of the subsequently bonded chip is gradually weakened as compared with the chip initially bonded. This causes the entire solder formed on the platform to oxidize under repeated heat treatment prior to bonding. In the prior art, in order to improve the above problems, there is a method of using scrubbing or ultrasonic waves, which can remove some of the oxide film on the surface of the solder by friction while the chip and the platform are in contact with each other. There was a limit. This is because a thin solder metal film having a thickness of 5 µm or less initially oxidizes only its surface, but gradually changes the trait of the entire film.

Meanwhile, another multiple flip chip bonding method for preventing oxidation of solder by repeated heat treatment shown in FIG. 2 is as follows. In the bonding method, when first bonding each chip to the platform, the solder is heated below the melting point so that the solder is not oxidized. After the completion of the first bonding of all chips, the bonding method simultaneously heats the chip and the platform to a temperature higher than that of the solder. To join.

That is, unlike conventional flip chip bonding, the platform 15 is heated above the temperature at which the solder 17 is melted (hereinafter, melting point), and the individual chips 11 and the platform 15 are bonded in a thermocompression bonding manner. In flip chip bonding, the process is divided into two stages. The first step is to align the chip 11 and the platform 15 with their respective alignment marks, and then load the chip 11 with the chip tool 21 below the temperature at which the solder 17 melts. Primary bonding to (15), and the second step is to repeat the above process according to the number of bonding required, and then bonded by heating the primary elements 11, 15 in the hot plate 25 above the melting point of the solder bonding To strengthen.

However, in this method, since the pressure is not applied to the chip and the substrate during the secondary bonding process in which the solder is melted, the quality of the bonding varies depending on the solder and chip plating states, that is, the concentration of the solder and the atmosphere / temperature conditions. If the bonding area is local, there is a problem in that the solder agglomerates at the contact portion and the equilibrium between the chip and the platform is broken, resulting in misalignment.

An object of the present invention for solving the above problems in the prior art is to provide a flip chip bonding method that is not affected by oxidation of the solder before and after bonding.

Another object of the present invention is to increase the number of optical active devices integrated in a single optical waveguide platform in the fabrication of optical modules to be used in the WDM-based passive optical subscriber network.

Still another object of the present invention is to reduce the manufacturing cost of a hybrid integrated optical module manufactured by an optical waveguide platform-based flip chip bonding method.

The flip chip bonding method of the present invention for achieving the above object is a method of bonding and maintaining the alignment between a plurality of optical active device chip and a single optical waveguide platform, forming a plating layer on a predetermined position on the optical active device chip step; Forming solder on the plating layer, the solder being melted at a predetermined melting point and acting as an adhesive material between the optical active device chip and the optical waveguide platform; Forming a solder pad made of a material easily bonded to the solder on the optical waveguide platform; And while maintaining the optical waveguide platform at a predetermined first temperature, the optical active elements to be bonded are sequentially heated to a predetermined second temperature to compress the optical waveguide platform to the optical waveguide platform. And bonding the device chip and the optical waveguide platform to each other, wherein the first temperature is lower than the melting point of the solder, and the second temperature is higher than the melting point of the solder.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, embodiments of the present invention illustrated below may be modified in many different forms, and the scope of the present invention is not limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

First, FIG. 3 is a cross-sectional view of an optical active device chip and an optical waveguide platform for flip chip bonding according to the present invention, and FIG. 4 illustrates a process of multiple flip chip bonding by applying the structure of FIG.

Referring to FIG. 3, the optical active device chip 31 is composed of a plurality of compound thin film layers (not shown), and the plating layer 32 is formed by performing gold plating on a surface bonded to the optical waveguide platform 35 for electrical connection. ). In addition, a backside (or backside) may be polished to facilitate scribing, and a backside plating layer (not shown) may be formed on the polished backside to apply a current having a polarity opposite to that of the plating layer 32. . Solder 33 for bonding to the optical waveguide platform is formed after the above-described process, it can be formed before the deposition of the anti-reflective or high reflection coating film deposited according to the scribing and the use of each device. The solder 33 may be formed by various methods such as thermal deposition, electron beam deposition, or electroplating. In this embodiment, a deposition technique using a lift-off process and a thermal evaporator is mainly used. The material of the solder may be various applications such as PbSn, AuSn, InPb. It is preferable that the solder is made of a conductor material so that the solder 33 serves not only to bond the optical waveguide platform and the optical active device chip but also to serve as a signal transmission path between the devices. In this case, the plating layer The 32 may be connected to an external input / output wire of the optical active device chip 31, and the solder pad 36 may be connected to a wire in the optical waveguide platform 35.

On the other hand, the optical waveguide platform is manufactured by a general process technology, the detailed process will be described as follows.

On the optical waveguide platform 35 of silica or polymer material formed on the silicon substrate, a solder pad 36 is formed at a wire position corresponding to the electrode of the optical active element chip 31. In this embodiment, an under bump (UBM) Metal film 36 was formed. The UBM film 36 may be formed by, for example, thermally depositing or electron beam depositing a metal such as Ti / Pt / Au or Cr / Ni / Au, and using a pattern such as lift-off. To form. In the case where the solder formed on the chip is an alloy of AuSn, the thickness of gold (Au) deposited at this time is preferably within 1 μm. In addition, a solder dam (not shown) may be formed by using a nitride film in order to prevent problems such as short circuit or weakening of a bond strength by flowing out of an area outside a predetermined area in the platform during melting of the solder 33.

The plating layer forming step and the solder forming step are processes of forming an adhesive structure of the photoactive device chip 31 shown in the upper part of FIG. 3, and the solder pad forming step is an optical waveguide platform 35 shown in the lower part of FIG. 3. ) To form an adhesive structure. Although the solder pad forming step is described as the last in the order of expression, the solder pad forming step may be performed before the plating layer forming step, and the plating layer forming step or the solder formation is performed because the processes are performed on independent devices. It may be performed while the step is being performed.

FIG. 4 illustrates the multiple flip-chips of a multi-channel optical module, such as a WDM-PON, on a single platform using the structures of the optical active device chip 31 and the optical waveguide platform 35 shown in FIG. The process of joining the chip is shown. The optical active element chip 31 and the optical waveguide platform 35 are respectively fixed to the metal structures 41 and 45 which are capable of conducting heat required for bonding from a heat source by vacuum suction or the like. Various heating devices may be used as the heat sources 43 and 47, but halogen lamps are generally used, and heat is evenly transmitted to an area where the structures 41 and 45 are in contact with the chip 31 and the platform 35. do. The alignment between the chip 31 and the platform 35 depends on the type of flip chip bonder but generally consists of identifying the alignment marks located on each by a camera equipped with an optical microscope between the two. After the alignment is completed, the structure 41 fixing the chip is brought down to the position in contact with the platform 35, the chip 31 is bonded on the platform 35 with a set load (or pressure). If the load is applied while the solder is solid, the initial alignment can be misaligned, so the heating required for melting the solder should be done at the initial point when the solder of the chip 31 touches the UBM film of the platform 35. The temperature at this time increases the temperature above the melting point of the upper portion of the junction so that the solder on the chip 31 receives sufficient heat, and the solder of the chip 31 that is already bonded is not deformed by the secondary heat. 35) is heated to a temperature below the melting point. For example, when the solder is an alloy of AuSn, the chip 31 may be heated to 310 ± 10 ° C and the platform 35 to 280 ± 10 ° C.

The present invention has been described in detail with reference to preferred embodiments, but the present invention is not limited to the above embodiments, and various modifications can be made by those skilled in the art within the scope of the technical idea of the present invention. Do.

In the multiple flip chip bonding method according to the present invention, a solder is formed on an optical active device chip and a UBM film for solder pad is formed on an optical waveguide platform to prevent oxidation of the solder due to repeated heat treatment expected when bonding a plurality of chips. There is no limit on the number of bonding, and there is an effect of facilitating the fabrication of multi-channel optical modules such as WDM-PON.

In addition, in the multiple flip chip bonding method according to the present invention, since solder is separate for each individual chip, even if the number of times of bonding is increased by repeated heat treatment, the initial temperature setting of the solder can be maintained.

In addition, the multiple flip chip bonding method according to the present invention has the effect that the melting point of the solder of the bonded chip is higher than the initial melting point and is not affected by the repeated heat treatment, so that problems such as misalignment of the aligned state do not occur. .

In addition, the multiple flip chip bonding method according to the present invention, by including a relatively expensive solder process in the manufacturing process of the chip with high mass production rate, it is possible to reduce the manufacturing cost of the hybrid integrated optical module bonded multiple flip chips There is.

In addition, the multiple flip chip bonding method according to the present invention does not require expensive additional equipment such as a laser heating device for local heating to prevent oxidation of the solder or an ultrasonic device for removing the oxide from the solder, thereby reducing the bonding process cost. It can be effective.

Claims (4)

A method of bonding a plurality of optically active chip and a single optical waveguide platform, the method comprising: Forming a plating layer on the plurality of optically active device chips; Forming a solder on the plating layer; Forming a solder pad on the optical waveguide platform; While maintaining the optical waveguide platform at a predetermined first temperature, the plurality of optical active devices to be bonded are sequentially heated to a predetermined second temperature to be compressed onto the optical waveguide platform to compress the plurality of optical active devices. Interconnection of the chip and the optical waveguide platform Chip bonding method comprising a. The method of claim 1, The first temperature is lower than the melting point of the solder, And the second temperature is higher than the melting point of the solder. The method according to claim 1 or 2, The plating layer is connected to the external input and output wiring of the optical active device chip, The solder pad is connected to wiring in the optical waveguide platform, And the solder is a conductor material. The method according to claim 1 or 2, Heating to the second temperature for melting the solder occurs at an initial point in time when the solder on the optically active element chip contacts the solder pad of the optical waveguide platform.

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