Optical module

Abstract

This optical module is provided with: an optical semiconductor chip, a sub-carrier, and an electrode junction part that electrically connects the optical semiconductor chip and the sub-carrier. The optical semiconductor chip is provided with: an optical waveguide, a first stress relaxation groove and a second stress relaxation groove that are disposed along the optical waveguide so as to sandwich the optical waveguide when viewed from above the optical semiconductor chip, and p-type electrodes and n-type electrodes that are disposed so as to sandwich the first stress relaxation groove and the second stress relaxation groove. The electrode junction part is connected to the p-type electrodes and the n-type electrodes, and the first stress relaxation groove and the second stress relaxation groove are each deeper than the core part of the optical waveguide.

Description

Optical Module

[0001] The present disclosure relates to optical modules.

[0002] Non-Patent Document 1 discloses a distributed feedback (DFB) laser chip with both p-type and n-type electrodes on its top surface. The chip is flip-chip mounted on a subcarrier, and an underfill material is poured into the gap between the subcarrier and the chip to secure the chip, forming an optical module. The underfill material prevents breakage of the electrode junction due to stress caused by the difference in thermal expansion coefficient between the chip and the subcarrier.

[0003] A. Marinins et. al., “Wafer-Scale Hybrid Integration of InP DFB Lasers on Si Photonics by Flip-Chip Bonding With sub-300 nm Alignment Precision,” J of Sel. In Quantum Electronics, vol. 29, no. 3, 8200311.

[0004] According to the optical module described in Non-Patent Document 1, bonding methods such as soldering, conductive adhesives, and ultrasonic bonding are used to electrically bond the chip to the subcarrier. These bonding methods are performed at high temperatures, and then when the temperature returns to room temperature, the bond between the chip and the subcarrier shrinks, which can cause stress. This poses a problem in that the stress generated at the bond can cause changes in chip characteristics, such as a shift in the laser oscillation wavelength.

[0005] The present disclosure has been made in view of the above-mentioned problems, and has an object to provide an optical module in which fluctuations in chip characteristics are suppressed when the chip is electrically connected to a subcarrier.

[0006] In order to solve the above-mentioned problems, an optical module according to one aspect of the present disclosure includes an optical semiconductor chip, a subcarrier, and an electrode junction that electrically connects the optical semiconductor chip and the subcarrier. The optical semiconductor chip includes an optical waveguide, first and second stress relief grooves that are arranged along the optical waveguide so as to sandwich the optical waveguide when viewed from above the optical semiconductor chip, and a p-type electrode and an n-type electrode that are arranged on either side of the first and second stress relief grooves. The electrode junction is connected to the p-type electrode and the n-type electrode, and the first and second stress relief grooves are each deeper than a core portion of the optical waveguide.

[0007] According to the present disclosure, it is possible to provide an optical module in which variations in chip characteristics are suppressed when the chip is electrically connected to a subcarrier.

[0008] Fig. 1 is a top view of an optical semiconductor chip included in an optical module according to a first embodiment of the present disclosure. Fig. 2 is a cross-sectional view of the optical semiconductor chip of Fig. 1 taken along line A-A. Fig. 3 is a layout diagram of electrode junctions for the optical semiconductor chip of Fig. 1. Fig. 4 is a side view of the optical module according to the first embodiment of the present disclosure. Fig. 5 is a top view of an optical semiconductor chip included in an optical module according to a second embodiment of the present disclosure.

[0009] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and redundant description will be omitted.

[0010] [Description of First Embodiment] Fig. 1 is a top view of an optical semiconductor chip included in an optical module according to a first embodiment of the present disclosure. Fig. 2 is a cross-sectional view of the optical semiconductor chip of Fig. 1 taken along line A-A. Fig. 3 is a layout diagram of electrode bonding sections for the optical semiconductor chip of Fig. 1. Fig. 4 is a side view of the optical module according to the first embodiment of the present disclosure. The optical module includes an optical semiconductor chip 10, a subcarrier 20, and electrode bonding sections.

[0011] The optical semiconductor chip 10 is at least one of a directly modulated DFB laser chip and an electroabsorption optical modulator integrated laser chip. For example, the optical semiconductor chip 10 is a chip fabricated on an indium phosphide (InP) substrate. Light output from the optical semiconductor chip 10 (output light R1) is introduced into a lightwave circuit (not shown). The optical semiconductor chip 10 is not limited to the examples given here.

[0012] The subcarrier 20 incorporates wiring for supplying power to the optical semiconductor chip 10 or wiring for transmitting signals from the optical semiconductor chip 10. For example, the subcarrier 20 has an aluminum nitride substrate and a gold wiring pattern. The subcarrier 20 is not limited to the example given here.

[0013] Alternatively, the thermal expansion coefficient of the subcarrier 20 may be equal to that of the optical semiconductor chip 10. This prevents breakage of the electrode junction due to stress caused by the difference in thermal expansion coefficient between the optical semiconductor chip 10 and the subcarrier 20.

[0014] 1 and 2, the optical semiconductor chip 10 includes an optical waveguide WG, stress relief groove TP2 (first stress relief groove), stress relief groove TN2 (second stress relief groove), p-type electrodes EP1, EP2, and n-type electrodes EN1, EN2. The stress relief grooves TP2, TN2 are arranged along the optical waveguide WG so as to sandwich the optical waveguide WG when viewed from above the optical semiconductor chip 10. In FIG. 1, the stress relief grooves TP2, TN2 are arranged along the direction in which the optical waveguide WG extends. The stress relief groove TP2 may also serve as a groove for drawing out the p-type electrode EP2. The stress relief groove TN2 may also serve as a groove for drawing out the n-type electrode EN2.

[0015] The p-type electrode EP2 is arranged on the upper surface of the optical semiconductor chip 10, on the opposite side of the optical waveguide WG across the stress relief groove TP2. The n-type electrode EN2 is arranged on the upper surface of the optical semiconductor chip 10, on the opposite side of the optical waveguide WG across the stress relief groove TN2.

[0016] 2, in a cross section of the optical semiconductor chip 10 perpendicular to the direction in which the optical waveguide WG extends, it can be seen that the n-type electrode EN2, stress relief groove TN2, optical waveguide WG, stress relief groove TP2, and p-type electrode EP2 are arranged in this order. The optical waveguide WG has a core portion CR, and both side surfaces of the core portion CR are filled with a semi-insulating semiconductor SR.

[0017] The p-type electrodes EP1, EP2 and the n-type electrodes EN1, EN2 are electrically connected to wiring built into the subcarrier 20 via electrode junctions. Here, the electrode junctions are made of at least one of solder, a conductive adhesive, and a metal.

[0018] For example, a conductive adhesive is applied to these electrodes of the optical semiconductor chip 10, and the optical semiconductor chip 10 is flip-chip mounted on the subcarrier 20. The p-type electrodes EP1, EP2 and the n-type electrodes EN1, EN2 are electrically connected to wiring built into the subcarrier 20. In addition to the conductive adhesive, the electrical connection may be achieved using solder, metal, or ultrasonic bonding.

[0019] Additionally, the stress relief grooves TP2 and TN2 are each formed deeper than the core portion CR of the optical waveguide WG. Furthermore, the stress relief grooves TP2 and TN2 are formed longer along the extension direction of the optical waveguide WG than the laser portion (not shown) of the optical semiconductor chip 10.

[0020] In order to confirm the effect of providing the stress relief grooves TP2, TN2, an optical semiconductor chip 10 without the stress relief grooves TP2, TN2 was prepared as a comparative example. The optical semiconductor chip 10 shown in FIGS. 1 and 2 was then compared with the comparative example.

[0021] The conditions for comparison were as follows: the optical semiconductor chip 10 was an electroabsorption optical modulator integrated laser chip fabricated on an InP substrate, the subcarrier 20 had an aluminum nitride substrate, and gold-tin solder was used for the electrode junctions.

[0022] The optical semiconductor chip 10 was mounted so that the top surface on which the electrodes were located was in contact with the wiring side surface of the subcarrier 20, and the optical module was assembled by heating it to 320 degrees, which is above the melting point of the gold-tin solder, holding it there for about 30 seconds, and then cooling it to room temperature.

[0023] 1 and 2, the oscillation wavelength was measured before mounting on the subcarrier 20, and was found to be 1308.1 nm at a chip temperature of 25°C and a laser current of 100 mA. Furthermore, the oscillation wavelength was measured after mounting on the subcarrier 20, and was found to be 1308.3 nm under the same operating conditions. In other words, the deviation in the oscillation wavelength was 0.2 nm.

[0024] On the other hand, when the oscillation wavelength of the comparative example was measured before being mounted on the subcarrier 20, it was 1306.5 nm at a chip temperature of 25°C and a laser current value of 100 mA. Furthermore, when the oscillation wavelength was measured after being mounted on the subcarrier 20, it was 1305.0 nm under the same operating conditions. In other words, the deviation in the oscillation wavelength was 1.5 nm.

[0025] From the above results, it was confirmed that the optical semiconductor chip and optical module according to this embodiment are able to suppress fluctuations in oscillation wavelength caused by stress, compared to the optical semiconductor chip according to the comparative example.

[0026] [Description of Second Embodiment] Figure 5 is a top view of an optical semiconductor chip included in an optical module according to a second embodiment of the present disclosure. Compared to Figure 1, the optical semiconductor chip included in the optical module according to this embodiment further includes stress relief groove TP1 (first stress relief groove) and stress relief groove TN1 (second stress relief groove). Stress relief groove TP1 may also serve as a groove for extracting the p-type electrode EP1. Stress relief groove TN1 may also serve as a groove for extracting the n-type electrode EN1.

[0027] In order to confirm the effect of providing the stress relief grooves TP1, TN1, an optical semiconductor chip without the stress relief grooves TP1, TN1 was prepared as a comparative example. Then, the optical semiconductor chip 10 shown in FIG. 5 was compared with the comparative example.

[0028] The conditions for comparison were as follows: the optical semiconductor chip 10 was an electroabsorption optical modulator integrated laser chip fabricated on an InP substrate, the subcarrier 20 had an aluminum nitride substrate, and gold bumps and conductive adhesive were used for the electrode junctions.

[0029] After applying the conductive adhesive to the wiring surface of the subcarrier 20, the optical semiconductor chip 10 was mounted so that the top surface on which the electrodes were arranged was in contact with the wiring side surface of the subcarrier 20. After that, the optical module was assembled by holding it at 150 degrees, which is the hardening temperature of the conductive adhesive, for about 20 minutes and then cooling it.

[0030] 5, the oscillation wavelength was measured before mounting on the subcarrier 20, and was found to be 1550.4 nm at a chip temperature of 25°C and a laser current of 120 mA. Furthermore, the oscillation wavelength was measured after mounting on the subcarrier 20, and was found to be 1550.7 nm under the same operating conditions. In other words, the deviation in the oscillation wavelength was 0.3 nm.

[0031] On the other hand, when the oscillation wavelength of the comparative example was measured before being mounted on the subcarrier 20, it was 1545.5 nm at a chip temperature of 25°C and a laser current value of 120 mA. Furthermore, when the oscillation wavelength was measured after being mounted on the subcarrier 20, it was 1546.9 nm under the same operating conditions. In other words, the deviation in the oscillation wavelength was 1.4 nm.

[0032] From the above results, it was confirmed that the optical semiconductor chip and optical module according to this embodiment are able to suppress fluctuations in oscillation wavelength caused by stress, compared to the optical semiconductor chip according to the comparative example.

[0033] The extinction characteristics of the EA modulator included in the optical semiconductor chip 10, which is an electroabsorption optical modulator integrated laser chip, were measured.

[0034] 5, before being mounted on the subcarrier 20, the optical output power was measured at a chip temperature of 25°C, a laser current value of 120 mA, and EA modulator bias voltages of 0 V and -2.5 V, and the difference in optical output power was 12.0 dB. After being mounted on the subcarrier 20, the difference in optical output power was 11.9 dB under the same operating conditions.

[0035] On the other hand, for the comparative example, before being mounted on the subcarrier 20, the difference in optical output power was 13.0 dB when the optical output power was measured at a chip temperature of 25°C, a laser current value of 120 mA, and bias voltages of 0 V and -2.5 V for the EA modulator. After being mounted on the subcarrier 20, the difference in optical output power was 14.0 dB under the same operating conditions.

[0036] From the above results, it was confirmed that, compared to the optical semiconductor chip of the comparative example, the optical semiconductor chip and optical module of this embodiment are able to suppress fluctuations in the extinction characteristics of the EA modulator caused by stress.

[0037] [Effects of the Embodiment] As described above in detail, the optical module according to the present embodiment includes an optical semiconductor chip, a subcarrier, and an electrode junction that electrically connects the optical semiconductor chip and the subcarrier. The optical semiconductor chip includes an optical waveguide, first stress relief grooves and second stress relief grooves that are arranged along the optical waveguide so as to sandwich the optical waveguide when viewed from above the optical semiconductor chip, a p-type electrode that is arranged on the top surface of the optical semiconductor chip on the opposite side of the first stress relief groove from the optical waveguide, and an n-type electrode that is arranged on the top surface of the optical semiconductor chip on the opposite side of the second stress relief groove from the optical waveguide. The electrode junction connects the p-type electrode and the n-type electrode, and the first stress relief groove and the second stress relief groove are each deeper than the core portion of the optical waveguide.

[0038] This makes it possible to provide an optical module in which fluctuations in chip characteristics are suppressed when the chip is electrically bonded to the subcarrier. In particular, by making the first stress relief groove and the second stress relief groove deeper than the core portion of the optical waveguide, stress generated at the electrode bonding portion is prevented from being transmitted to the core portion of the optical waveguide. As a result, fluctuations in chip characteristics are suppressed.

[0039] In the optical module according to this embodiment, the optical semiconductor chip may further include a laser section, and the first and second stress relief grooves may each be longer than the laser section. This prevents stress generated at the electrode junction from being transmitted to the laser section, thereby suppressing deviation in the laser oscillation wavelength.

[0040] Furthermore, in the optical module according to this embodiment, the optical semiconductor chip is at least one of a directly modulated DFB laser chip and an electro-absorption optical modulator integrated laser chip, thereby improving the high frequency characteristics of the directly modulated DFB laser chip and the electro-absorption optical modulator integrated laser chip and increasing the wavelength stability.

[0041] In the optical module according to this embodiment, both side surfaces of the core may be filled with a semi-insulating semiconductor. This insulates the core from the p-type and n-type electrodes located nearby. As a result, fluctuations in the characteristics of the core are suppressed.

[0042] Furthermore, in the optical module according to this embodiment, the p-type electrode and the n-type electrode are electrically connected to the wiring built into the subcarrier via electrode junctions, which may be made of at least one of solder, conductive adhesive, and metal, thereby ensuring electrical connection between the electrodes of the optical semiconductor chip 10 and the wiring built into the subcarrier.

[0043] In the optical module according to this embodiment, the thermal expansion coefficient of the subcarrier may be equal to that of the optical semiconductor chip. This reduces stress caused by the difference in thermal expansion coefficients between the optical semiconductor chip and the subcarrier when the optical semiconductor chip is mounted on the subcarrier. As a result, breakage of the electrode joint between the optical semiconductor chip and the subcarrier is suppressed.

[0044] Although the contents of the present disclosure have been described above based on the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which will be apparent to those skilled in the art. The descriptions and drawings that form part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0045] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description.

[0046] CR Core portion EN1, EN2 n-type electrodes EP1, EP2 p-type electrodes TP1, TP2, TN1, TN2 Stress relief groove WG Optical waveguide 10 Optical semiconductor chip 20 Subcarrier

Claims

1. An optical module comprising: an optical semiconductor chip; a subcarrier; and an electrode junction electrically connecting the optical semiconductor chip and the subcarrier, wherein the optical semiconductor chip comprises: an optical waveguide; first stress relief groove and second stress relief groove arranged along the optical waveguide so as to sandwich the optical waveguide when viewed from above the optical semiconductor chip; a p-type electrode arranged on the upper surface of the optical semiconductor chip, on the opposite side to the optical waveguide across the first stress relief groove; and an n-type electrode arranged on the upper surface of the optical semiconductor chip, on the opposite side to the optical waveguide across the second stress relief groove, wherein the electrode junction is connected to the p-type electrode and the n-type electrode, and the first stress relief groove and the second stress relief groove are each deeper than a core portion of the optical waveguide.

2. The optical module according to claim 1, wherein the optical semiconductor chip further comprises a laser portion, and the first stress relief groove and the second stress relief groove are each longer than the laser portion.

3. The optical module according to claim 1, wherein the optical semiconductor chip is at least one of a directly modulated DFB laser chip and an electric field absorption type optical modulator integrated laser chip.

4. An optical module according to claim 1, wherein both side surfaces of said core portion are filled with a semi-insulating semiconductor.

5. The optical module according to claim 1, wherein the p-type electrode and the n-type electrode are electrically connected to wiring built into the subcarrier via the electrode junction, and the electrode junction is at least one of solder, a conductive adhesive, and a metal.

6. An optical module according to any one of claims 1 to 5, wherein the thermal expansion coefficient of said subcarrier is equal to that of said optical semiconductor chip.

Images