The present invention relates to optical modules for sending and receiving optical signals that have excellent high frequency properties.
In recent years, optical fiber communication, which is capable of transmitting large volumes of information with little loss, has been practiced as an alternative to communications employing metallic cable or wireless media.
When video signals are received over an optical fiber, a light-receiving device serves as the light-receiving front end portion. Light-receiving devices are made of a light-receiving element such as a photodiode (PD) that receives optical signals and generates a small current corresponding to those signals, and an element that, once the small current that has been generated is converted into voltage, amplifies the signals up to a reception sensitivity required by a television receiver or the like that is connected at a later stage and demodulates them.
The frequency band of signals processed by such light-receiving devices for receiving video signals has become increasingly high in the case of CATV, for example, as the number of channels increases, and at present is approaching 1 GHz.
A conventional example of a device for optical communications having good high frequency properties in a system that employs optical fiber to distribute video signals for multiple channels is disclosed in JP 2001-345456A, and is a wideband light-receiving device in which a capacitive element with excellent low-distortion properties over a wide frequency band is provided internally in a substrate. In this wideband light-receiving device, semiconductor elements are flip-chip mounted onto a multilayer substrate in which a capacitive element is provided internally in order to reduce parasitic inductance, and as a result the semiconductor elements and the capacitive element can be connected very close to one another, thereby resulting in excellent high frequency properties.
However, the following problems occur if the capacitance of the capacitive element is increased in a conventional module for optical communications.
FIG. 22 is a cross-sectional view showing the configuration of a conventional module for optical communications. FIGS. 23A-B is a cross-sectional view showing the processes for manufacturing a separate conventional module for optical communications.
With the module for optical communications shown in FIG. 22, an optical element 201 and a semiconductor element 231 are flip-chip mounted to terminal electrodes 202 formed on the surface of a multilayer substrate 203 via bumps 207. Within the multilayer substrate 203, an upper electrode 205 and a lower electrode 206 sandwich a dielectric layer 209 and thereby form a capacitive element, and the upper electrode 205 and the lower electrode 206 are connected electrically to the terminal electrodes 202 through via holes 208. With such a configuration, the capacitive element can be formed inside the multilayer substrate 203 below the semiconductor element 231. Also, because the dielectric layer 209 is formed spanning the entire area of the multilayer substrate 203, there is no unevenness in the surface of the multilayer substrate 203, and the optical element 201 and the semiconductor element 231 can be stably flip-chip-mounted onto the multilayer substrate 203.
To increase the capacitance of the capacitive element, the dielectric layer 209 can be formed using a material with a high relative permittivity, but because the dielectric layer 209 spans a wide area, there is the problem that stray capacitance may occur at unnecessary areas and that cross-talk may occur in the internal wiring layer.
Accordingly, as disclosed in JP H06-164150A, for example, the dielectric film 209 can be formed in one region only and not formed over a wide area. With the module for optical communications shown in FIGS. 23A-B, the dielectric layer 209 is formed in one region using a material that differs from that of the multilayer substrate 203, and thus the capacitive element is formed only at necessary areas, and the semiconductor element 231 and the capacitive element are connected at a very close distance to one another.
With this configuration, however, unevenness results in the surface of the multilayer substrate 203 at areas where the dielectric layer 209 has not been formed because there are areas within the multilayer substrate 203 where the dielectric layer 209 has been formed. For that reason, if the semiconductor element 231 is flip-chip mounted onto the multilayer substrate 203 from the state shown in FIG. 23A, a gap occurs between the bump 207 of the semiconductor element 231 and the terminal electrodes 202 as shown in FIG. 23B, and thus the semiconductor element 231 cannot be stably flip-chip mounted.
Moreover, as shown in FIG. 24, unevenness in the surface makes it impossible to mount the optical element 201 at a predetermined location of the multilayer substrate 203 and position an optical fiber 230 by passive alignment in a predetermined location using a V-groove 271. That is, the height difference that occurs between the terminal electrodes 202a and 202b causes the optical element 201 to be tilted when flip-chip mounted. As a consequence the direction in which the laser is emitted diverges from the predetermined direction and optical coupling with the optical fiber 230, which is arranged in a predetermined position, cannot be obtained. It should be noted that the V-groove 271 is formed in a bench 261 and the optical module is mounted onto the bench 261 via a connection terminal 251.
More specifically, a vertical disparity of about 10 xcexcm occurs between the terminal electrode 202a and the terminal electrode 202b. For example, if the spacing between the bump 207a and the bump 207b in the direction of the optical axis is 200 xcexcm and the vertical disparity between the terminal electrode 202a and the terminal electrode 202b is 20 xcexcm, then an emission direction 241 of the optical element 201 is tilted with respect to an optical axis 242 of the optical fiber 230 by 5.7 degrees.
Light that is incident within 5.7 degrees of the optical axis 245 into an ordinary single-mode optical fiber 230 with a numerical aperture of 0.1 can be coupled. However, the light emitted from the optical element 201, which is a laser element, has a flare angle of a certain degree and its optical strength is in a Gaussian distribution with respect to the emission axis. Thus, a laser element that has a full width at half maximum of 15 degrees or more cannot be optically coupled with the optical fiber 230.
The present invention was arrived at in light of the foregoing problems, and it is an object thereof to provide an optical module that has good high frequency properties, in which an optical element and a semiconductor element, for example, are mounted stably onto a multilayer substrate.
An optical module of the present invention is provided with a substrate that includes an insulating layer, a passive element provided inside or on a surface of the insulating layer, and terminal electrodes formed on the surface of the insulating layer, and with at least one active element, which includes at least an optical element and is connected to the terminal electrodes on the substrate surface. The passive element has a dielectric layer, a resistive layer, or a magnetic layer, at least one of the terminal electrodes is connected to the passive element, and at least one of the at least one active element has a protruding electrode and is flip-chip mounted to the terminal electrodes on a principle face of the substrate via the protruding electrode. Taking a plane parallel to the principle face of the substrate as a projection plane, an area of orthographic projection of the dielectric layer, the resistive layer, or the magnetic layer is smaller than an area of orthographic projection of the principle face of the substrate, and the dielectric layer, the resistive layer, or the magnetic layer is formed such that the orthographic projection, with respect to the projection plane, of all the protruding electrodes of the at least one active element that is flip-chip mounted to the principle face of the substrate is included in the orthographic projection of the dielectric layer, the resistive layer, or the magnetic layer. It should be noted that a principle face is a surface of the substrate and represents the widest surface thereof.
Thus, unevenness can be prevented in areas where the protruding electrodes of the terminal electrodes are connected. For that reason, active elements can be stably flip-chip mounted onto the substrate. Also, the active elements formed on the primary face of the substrate and the passive element inside the substrate can be connected at a close distance, so that parasitic inductance can be reduced and the high frequency properties are excellent. In particular, in the frequency properties when light signals that are incident on or emitted from the optical element are converted into electrical signals, the cutoff frequency at which the conversion gain is halved is increased in frequency and thus the band can be widened.
Further, the at least one active element that has been flip-chip mounted can be present on only one principle face of the substrate.
Further, the at least one passive element that has been flip-chip mounted can be present on both principle faces of the substrate.
Further, the optical element further can include terminal electrodes on an end face of the substrate and an active element flip-chip mounted to these terminal electrodes.
Further, it is preferable that the total of a distance between the surface of the dielectric layer, the resistive layer, or the magnetic layer and a center of a region of contact between the protruding electrode of the optical element that has been flip-chip mounted to the principle face of the substrate and the terminal electrode, and a distance from a point where a perpendicular line passing through the center of the region of contact between the protruding electrode and the terminal electrode intersects with the surface of the dielectric layer, the resistive layer, or the magnetic layer to the end portion of the dielectric layer, the resistive layer, or the magnetic layer that is farthest from that point, is less than a distance corresponding to xc2xd the wavelength of the electrical signals that are processed by the optical element. Thus, the capacitive element that is formed by the dielectric layer, the resistive layer, or the magnetic layer can be kept from becoming inoperable.
Furthermore, it is preferable that the dielectric layer, the resistive layer, or the magnetic layer is formed independently at one or at each of a plurality of the terminal electrodes that are formed on the principle face of the substrate. Thus, the dielectric layer, the resistive layer, or the magnetic layer can be reduced in size, which allows costs to be reduced.
Further, the at least one active element includes a semiconductor element.
In addition, it is preferable that a region where the dielectric layer, the resistive layer, or the magnetic layer has not been formed is present in the region where the orthographic projection with respect to the projection plane of all the protruding electrodes of the semiconductor element that is arranged onto the principle face of the substrate is not formed. Thus, the degree of freedom for the wiring between the terminal electrodes for the semiconductor element on the substrate surface and the passive element that is internally provided in the substrate can be increased.
Further, it is preferable that a via conductor is formed in a region where the dielectric layer, the resistive layer, or the magnetic layer has not been formed. Thus, the via conductor is formed by a material with a high thermal conductivity and is arranged directly below the semiconductor element, so that heat from the semiconductor element can be dissipated efficiently.
Further, it is preferable that the passive element includes a pair of passive element electrodes formed sandwiching the dielectric layer, the resistive layer, or the magnetic layer, and that the pair passive element electrodes are formed perpendicular to the terminal electrodes and are separated into a plurality of units in the surface. Thus, a different voltage can be set for each terminal of the active element, and a bypass capacitor of any capacitance can be provided at each terminal, so that the high frequency properties of the semiconductor element can be improved even further.
Also it is preferable that the optical module further includes an optical waveguide for guiding light and a bench that has a groove for securing the optical waveguide. Thus, light can be transferred through the optical waveguide, and moreover, the optical waveguide and the optical element can be aligned easily with one another.
Further, the groove can secure the optical waveguide so that an optical axis of the optical waveguide is substantially parallel to a principle face of the substrate.
Alternatively, the groove can fasten the optical waveguide so that an optical axis of the optical waveguide is substantially perpendicular to a principle face of the substrate.
It is further preferable that the optical module further includes an optical waveguide for guiding light and a groove for securing the optical waveguide, and that the groove is formed on the substrate and fastens the optical waveguide so that an optical axis of the optical waveguide is substantially parallel to the principle face of the substrate. Thus, it is not necessary to prepare a bench, and thus costs can be reduced.
Furthermore, the dielectric layer, the resistive layer, or the magnetic layer can be formed on the surface of the substrate.
It is further preferable that the at least one active element includes an optical element and a semiconductor element, that the optical element is flip-chip mounted to the terminal electrodes on one principle face of the substrate, and that the semiconductor element is flip-chip mounted to the terminal electrodes on the other principle face of the substrate. Thus, the optical coupling portion of the optical element and the heat dissipation portion of the semiconductor element are spatially separated from one another, and thus the heat dissipation efficiency is good.
It is furthermore preferable that a mixture including an inorganic filler and a thermosetting resin composition is packed around the semiconductor element. Thus, heat conductivity and the heat dissipation efficiency are high.
In addition, the inorganic filler can include at least one of alumina, aluminum nitride, silicon nitride, beryllia (BeO), and silica.
Also, plural passive elements can be formed.
Furthermore, the optical element can be a light-receiving element or a light-emitting element.
Further, the optical element can be a light-receiving element, and the semiconductor element can be an amplifier element for amplifying signals of the light-receiving element.
Further, it is preferable that the light-receiving element is a rear face-illuminated photodiode, and that the semiconductor element is a transimpedance-type wideband amplifier.
Additionally, it is preferable that the optical element is a light-emitting element, and that the semiconductor element is a drive element for driving the light-emitting element.
Also, it is preferable that the light-emitting element is an end face-emitting laser diode or a surface-emitting laser diode, and that the semiconductor element is a laser drive element.
It is further preferable that the insulating layer of the substrate is a low sintering temperature glass ceramic with an inorganic sintered material as a primary component, and that the dielectric layer of the passive element includes a lead-based perovskite compound as a primary component.
In a further preferable aspect, the insulating layer of the substrate is a low sintering temperature glass ceramic with an inorganic sintered material as a primary component, and the resistive layer of the passive element includes RuO2 as a primary component.
It is further preferable that the at least one active element includes an optical element and a semiconductor element, that the optical element is flip-chip mounted to the terminal electrodes that are formed on an end face of the substrate, and that the semiconductor element is flip-chip mounted to the terminal electrodes that are formed on the principle face of the substrate. Thus, the optical element and the passive element in the substrate can be connected near one another, so that there are excellent high frequency properties and the optical fiber and the optical element in the optical module can be optically coupled with ease.
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