WO2004007386A2

WO2004007386A2 - Thermal barrier for an optical bench

Info

Publication number: WO2004007386A2
Application number: PCT/CA2003/001038
Authority: WO
Inventors: Johannes Boelen; Bin Wang
Original assignee: Alfalight, Inc.
Priority date: 2002-07-11
Filing date: 2003-07-09
Publication date: 2004-01-22
Also published as: AU2003250646A8; AU2003250646A1; WO2004007386A3

Abstract

The thermal barrier (12) is made of borosilicate glass. It is used with an optical bench (10), in particular an optical bench (10) of an optical module assembly (20). It provides thermal insulation during the alignment of an optical fiber (26). A method of thermally insulating an optical bench (10) from a substrate (32) using the thermal barrier (12) is also disclosed. The thermal barrier (12) reduces the heat transfer to neighboring components during the fiber alignment.

Description

THERMAL BARRIER FOR AN OPTICAL BENCH

In the field of fiber optics communications, there is considerable interest in producing low cost and efficient systems. Considering that assembling the individual electro-optic components of optical modules and the alignment of optical fibers therewith represent a preeminent portion of the production costs, improvements in the design of optical module assemblies are essential for achieving this goal.

Solder is generally used in the manufacture of optical module assemblies in order to attach the end of an optical fiber in front of the output of a laser chip. FIG. 4 schematically illustrates a typical example of a conventional optical module assembly (100) in which solder is used. The assembly (100) comprises a semiconductor laser chip (102) mounted onto a chip carrier (104). The tip (108) of the optical fiber (106) is coupled to the output of the laser chip (102). The end section of the optical fiber (106) is attached to a small carrier, which is referred to as the optical bench (110). Solder (112) is used to attach the optical fiber (106) to the optical bench (110). The chip carrier (104) and the optical bench (110) are mounted on a substrate (114).

During the fiber alignment, heating power is applied to maintain the solder (112) at or above its melting point. In many circumstances it would be preferable to optimize the alignment of the optical fiber (106) to the output of the laser chip (102) whilst the solder (112) is still molten. However, because the optical bench (110) has usually both a high thermal conductivity and a low thermal mass, excess heat rapidly spreads to neighboring components, including the laser chip (102). This additional heat may cause the laser chip (102) to overheat up to the point where it no longer functions in a manner that would permit optimization of the alignment.

Another possible problem with a conventional optical module assembly (100) is that the die bonding between the optical bench (110) and the underlying substrate (114) can cause high component stresses when substantial local heating power is applied. Ultimately, this may result in the cracking of the optical bench (110). The reason is that the substrate (114) and the optical bench (110) are typically made of materials having a relatively different coefficient of thermal expansion (CTE). The large temperature gradient and CTE mismatch between the two components create high material stresses when the components are heated. This may ultimately cause a failure of the components and thus of the optical module assembly.

A conventional optical module assembly may be further equipped with a Thermo- Electric Cooler (TEC) located under its substrate. A potential problem with such optical module assembly is that if the excess heat is more than the TEC is capable of pumping away, the optimization of the alignment cannot be performed. In worst cases, overheating may cause the solder of the TEC elements to degrade, reflow or crack, thereby causing a degradation or failure thereof. There are thus possible problems if the TEC is overheated during the optical fiber alignment.

Various modifications to the design of optical module assemblies have been considered in the past in an attempt to provide some forms of thermal shielding. One example was to have a thick layer of grown silicon dioxide under the optical bench when the latter is made of silicon. However, this was found to cause even higher material stresses. Another proposed solution that was considered involved having channels under the optical bench to decrease the heat transfer. However, these channels increased part complexity, fragility and cost.

In view of the above, there was thus a need to improve the design of an optical module assembly such that the heating of the solder during the alignment be substantially prevented from affecting the laser chip. In the case of an optical module assembly provided with a TEC, the TEC temperature should remain low enough so it can be used as a cooling medium during the optical fiber alignment. Briefly stated, the new design that is hereby proposed with the present invention consists in providing a thermal barrier for the optical bench, the thermal barrier being made of borosilicate glass.

Another aspect of the present invention is to provide an optical module assembly for use with an optical fiber, the optical module comprising: a substrate; a laser chip carrier mounted on the substrate; a laser chip mounted on the laser chip carrier, the laser chip having an output on one side thereof; and an optical bench mounted on the substrate, the optical bench being mounted on the side of the output of the laser chip. The optical module assembly is characterized in that it further comprises a thermal barrier made of borosilicate glass provided between the optical bench and the substrate.

A further aspect of the present invention is to provide a method of thermally insulating an optical bench from a substrate, the method being characterized in that it comprises: providing a thermal barrier made of borosilicate glass; and bonding the thermal barrier between the optical bench and the substrate.

Various other aspects and advantages of the present invention are disclosed in the following detailed description.

The present description makes reference to the appended figures in which:

FIG. 1 is a schematic perspective view of an example of an optical bench with a thermal barrier in accordance with a preferred embodiment of the present invention.

FIG. 2 is a schematic side view of the optical bench and the thermal barrier of FIG. 1 used on an optical module assembly.

FIG. 3 shows the optical module assembly of FIG. 2 provided with a thermo-electric cooler (TEC). FIG. 4 is a schematic side view of an example of a conventional optical module assembly.

FIG. 1 schematically shows an example of an optical bench (10) with a thermal barrier (12) in accordance with a preferred embodiment of the present invention. It should be understood that the present invention is not limited to this precise embodiment and that various changes and modifications may be effected therein without departing from the scope of the present invention, as defined by the appended claims.

FIG. 1 shows the optical bench (10) in the form of a rectangular part. The optical bench (10) is preferably made of silicon (Si). The underside of the optical bench (10) is bounded to the thermal barrier (12). In accordance with the present invention, this thermal barrier (12) is made of borosilicate glass (also known as Pyrex®). Borosilicate glass has a very low thermal conductivity. It was found that it can serve as an efficient thermal barrier so that very little heat transfer to surrounding components occurs when heat is applied to the solder during the fiber alignment procedure.

FIG. 2 schematically illustrates an optical module assembly (20) provided with the optical bench (10) and the thermal barrier (12). The assembly (20) comprises a semiconductor laser chip (22) mounted onto a chip carrier (24). The tip (28) of an optical fiber (26) is coupled to the output of the laser chip (22). The end section of the optical fiber (26) is attached to the optical bench (10) by means of an amount of solder (30). The chip carrier (24) and the optical bench (10) are mounted on a substrate (32).

During the fiber alignment, heating power is applied to maintain the solder (30) at or above its melting point. The presence of the thermal barrier (12) prevents the excess heat to rapidly spread to the neighboring components, thereby causing damages of these components or lowering their performances. Another important advantage of using the thermal barrier (12) made of borosilicate glass is that this material has a Coefficient of Thermal Expansion (CTE) which is acceptably close to those of the materials generally used for the optical bench (10) and for the substrate (32). As aforesaid, the material preferably used for the optical bench (10) is silicon. As for the substrate (32), it is preferably made of aluminum nitride. Furthermore, borosilicate glass has a low modulus of elasticity compared to silicon, thereby allowing it to better accommodate any residual assembly and fiber alignment stresses.

Preferably, as shown in FIG. 1 , the optical bench (10) is provided with a thin metallization layer (14) of the top surface thereof. The metallization layer (14) will allow the solder to optimally adhere at areas where it is applied. FIG. 1 also shows a V-shaped groove (16) machined on the top surface of the optical bench (10). The V-shaped groove (16) will receive the optical fiber (26).

Advantageously, the thermal barrier (12) can be processed in the same manner as a silicon wafer. The borosilicate glass layer is preferably combined to the silicon layer at the wafer level to save process cost. Nevertheless, it is also possible to combine them at the component level.

Various techniques can be used to bond the borosilicate glass layer to the silicon layer. The preferred embodiment is the anodic wafer bonding. In this case, the melting temperature at the joint would be well above any of the process temperatures. Another advantage is that etching, machining, metallization and other treatments can be performed at the wafer level before or after the anodic bonding process, as the manufacturing process demands. Extra wafer processing costs are amortized over a large number of yielded components. Component handling is kept until after dicing.

As can be appreciated, the borosilicate glass layer provides a highly suitable thermal barrier (12) for an optical bench (10). Numerous advantages are resulting from this new design in addition to the reduction of the heat transfer to the laser chip (22) or even to a TEC (34), as shown in FIG. 3. For instance, it was also found that the heating power to be supplied for keeping the solder (30) at or above its melting point can be reduced by a factor of up to 10.

Claims

CLAIMS.

1. A thermal barrier (12) for an optical bench (10), the thermal barrier (12) being characterized in that it is made of borosilicate glass.

2. The thermal barrier (12) according to claim 1 , characterized in that the optical bench (10) is made of silicon.

3. The thermal barrier (12) according to claim 1 or 2, characterized in that it is provided under the optical bench (10), the thermal barrier (12) separating the optical bench from a substrate (32).

4. The thermal barrier (12) according to claim 3, characterized in that the substrate (32) is made of aluminum nitrate.

5. An optical module assembly (20) for use with an optical fiber (26), the optical module assembly (20) comprising: a substrate (32); a laser chip carrier (24) mounted on the substrate (32); a laser chip (22) mounted on the laser chip carrier (24), the laser chip (22) having an output on one side thereof; and an optical bench (10) provided on the substrate (32), the optical bench (10) being mounted on the side of the output of the laser chip (22); the optical module assembly (20) being characterized in that it further comprises: a thermal barrier (12) made of borosilicate glass provided between the optical bench (10) and the substrate (32).

6. The optical module assembly according to claim 5, characterized in that it further comprises a thermo-electric cooler (34) provided under the substrate (32).

7. The optical module assembly according to claim 5 or 6, characterized in that the optical bench (10) is made of silicon.

8. The optical module assembly (20) according to claim 5, 6 or 7, characterized in that the substrate (32) is made of aluminum nitrate.

9. The optical module assembly (20) according to any one of claims 5 to 8, characterized in that the optical bench (10) further comprises a metallization layer (14) on a top surface thereof.

10. The optical module assembly (20) according to any one of claims 5 to 9, characterized in that the optical bench (10) further comprises a V-shaped groove (16) machined on a top surface thereof.

11. A method of thermally insulating an optical bench (10) from a substrate (32), the method being characterized in that it comprises: providing a thermal barrier (12) made of borosilicate glass; and bonding the thermal barrier (12) between the optical bench (10) and the substrate (32).

12. The method according to claim 11 , characterized in that the bounding of the thermal barrier (12) comprises an anodic wafer bounding at a wafer level.

13. The method according to claim 11 or 12, characterized in that the optical bench (10) is made of silicon.

14. The method according to any one of claims 11 to 13, characterized in that the substrate (32) is made of aluminum nitrate.

15. The method according to any one of claims 11 to 14, characterized in that it further comprises: providing a metallization layer (14) on a top surface of the optical bench (10).

16. The method according to any one of claims 11 to 15, characterized in that it further comprises: machining a V-shaped groove (16) on a top surface of the optical bench (10).