MICROFABRICATED OPTICAL APPARATUS

Abstract

A microfabricated optical apparatus that includes a light source driven by a waveform, a turning mirror, and a beam shaping element, wherein the waveform is delivered to the light source by at least one through silicon via.

Claims

1. A microfabricated optical apparatus fabricated on a silicon substrate, comprising: a light source driven by a signal and disposed on the silicon substrate, wherein the light source generates optical radiation; wherein the signal is delivered to the light source by at least one through silicon via (TSV) which extends through a thickness of the silicon substrate.

2. The microfabricated optical apparatus of claim 1, further comprising a lid wafer with a device cavity formed therein, wherein the device cavity encapsulates the optical apparatus.

3. The microfabricated optical apparatus of claim 2, wherein the signal is a direct current electrical signal which is applied to the through silicon via.

4. The microfabricated optical apparatus of claim 1, further comprising: a device which modulates at least one of a frequency and an amplitude, to encode the optical radiation emitted from the light source with an information signal.

5. The microfabricated optical apparatus of claim 2, further comprising a Faraday rotator also disposed within the device cavity.

6. The microfabricated optical apparatus of claim 1, wherein the light source is at least one of a light emitting diode, a laser diode, an edge emitting laser diode, a laser diode, and a vertical cavity surface emitting laser.

7. The microfabricated optical apparatus of claim 2, wherein the optical radiation exits the device cavity through a roof of the lid wafer, in a substantially parallel direction relative to the through silicon via.

8. The microfabricated optical apparatus of claim 2, wherein the optical radiation exits the device cavity through the substrate, in a substantially parallel direction relative to the through silicon via.

9. The microfabricated optical apparatus of claim 2, wherein the optical radiation exits the device cavity through a sidewall of the device cavity, in a direction substantially orthogonal to the through silicon via.

10. The microfabricated optical apparatus of claim 2, wherein the device cavity encapsulates a plurality of light sources.

11. The microfabricated optical apparatus of claim 2, further comprising a beam shaping a lens formed in a roof of the device cavity and from material of the lid wafer.

12. The microfabricated optical apparatus of claim 2, further comprising a reflective film deposited on a sidewall of the device cavity, wherein the sidewall is inclined with respect to a surface of the substrate by about 50 to 60 degrees.

13. The microfabricated optical apparatus of claim 1, further comprising a reflective film deposited on an inclined surface of an optical element located within the device cavity.

14. A method for microfabricating an optical apparatus on a silicon substrate, comprising: forming a device cavity in a lid wafer; forming a through silicon via through the silicon substrate; disposing a light source driven by a waveform which generates optical radiation on the silicon substrate, and coupling the light source electrically to the through silicon via; disposing a beam shaping element on the silicon substrate; disposing a turning surface which redirects the beam of light; and bonding the substrate to the lid wafer to encapsulate the optical apparatus in a substantially hermetic device cavity.

15. The method of claim 14, wherein forming the through silicon via comprises: etching a blind trench into a front side of the substrate leaving residual substrate material; coating the trench with an insulating material; depositing a conductive material in the blind trench; and removing the residual substrate material from a backside of the substrate to form the via.

16. The method of claim 14, wherein bonding the silicon substrate to the lid wafer comprises bonding the silicon substrate to the lid wafer with a low temperature metal alloy bond.

17. The method of claim 14, further comprising: forming a lens in a roof of the device cavity, on an outside surface.

18. The method of claim 14, further comprising: forming a lens in a roof of the device cavity, on an inside surface.

19. The method of claim 14, wherein forming the device cavity comprises forming the device cavity with anisotropic etching, leaving inclined sidewalls in the device cavity inclined at angles of about 50 to 60 degrees with respect to a surface of the lid wafer.

20. The method of claim 19, further comprising: depositing a reflective surface onto at least one inclined sidewall of the device cavity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Various exemplary details are described with reference to the following figures, wherein:

[0017] FIG. 1 is a schematic illustration of a prior art microfabricated optical apparatus;

[0018] FIG. 2 is a schematic, cross sectional illustration of a first embodiment of a microfabricated optical apparatus;

[0019] FIG. 3 is a schematic, cross sectional illustration of a second embodiment of a microfabricated optical apparatus;

[0020] FIG. 4 is a schematic, cross sectional illustration of a third embodiment of a microfabricated optical apparatus;

[0021] FIG. 5 is a schematic, cross sectional illustration of a fourth embodiment of a microfabricated optical apparatus;

[0022] FIG. 6 is a schematic, cross sectional illustration of a fifth embodiment of a microfabricated optical apparatus.

[0023] FIG. 7 is a schematic, cross sectional illustration of a sixth embodiment of a microfabricated optical apparatus with a plurality of light sources; and

[0024] FIG. 8 is a plan view of a substrate with multiple optical apparatuses fabricated thereon.

DETAILED DESCRIPTION

[0025] The systems and methods described herein may be particularly applicable to microfabricated optical tables, wherein small optical devices are formed on a substrate surface and enclosed with a lid wafer. The optical devices may include light sources such as light emitting diodes (LED's), beam shaping structures such as lenses and turning mirrors, and polarization altering devices such as Faraday rotators and optical isolators. After fabrication, these devices may be enclosed with a lid wafer to protect them in an encapsulated device cavity. Some devices, such as optical detectors and optical or laser emitters, may require a vacuum or low moisture environment, such that the device cavity may need to be substantially hermetically sealed. The signal and power lines may be delivered to the sealed device cavity with a through substrate via (TSV). The TSV may have a bonding pad on one side of the substrate, and a conductive line leading to the device within the device cavity.

[0026] Through substrate vias may be particularly convenient for MEMS devices, because they may allow electrical access to the encapsulated devices. Without such through holes, electrical access to the MEMS device may have to be gained by electrical leads routed under the lid wafer which is then substantially hermetically sealed. It may be problematic, however, to achieve a substantially hermetic seal over terrain that includes the electrical leads unless more complex and expensive processing steps are employed. This approach also makes radio-frequency applications of the device limited, as electromagnetic coupling will occur from the metallic bondline residing over the normally oriented leads. Substantially hermetic is used herein, should be understood to provide a barrier against moisture penetration, and/or capable of maintaining vacuum to within about the 10 Torr range.

[0027] The systems and methods described herein may be particularly applicable to vacuum encapsulated optical tables, such as an LED, shaping lens, rotator/isolator and turning mirror, all enclosed in the device cavity.

[0028] The prior art is illustrated in FIG. 1, and exemplary embodiments of the novel optical apparatuses are illustrated in FIGS. 2-8.

[0029] FIG. 1 shows a prior art example of an microfabricated optical table. As can be seen in FIG. 1, the output of laser light source 1 may be shaped by a ball lens 2 and then through Faraday rotator 3. A Faraday rotator 3 s an optical device that rotates the polarization of light due to the Faraday effect, which in turn is based on a magnetooptic effect. The Faraday rotator 3 in combination with a quarter wave plate outside the cavity, may provide optical isolation. The beam of light then impinges on a turning mirror 5 which redirects the light in a direction normal to the substrate, shown downward in FIG. 1. The light may pass through the device substrate 7 on which the aforementioned devices are fabricated. In FIG. 1, the turning mirror is a discrete structure 5, encapsulated in the device cavity along with the other components.

[0030] One of the problems with the device shown in FIG. 1 is that the leads that drive the laser emitter are necessarily routed under the bond lines that bond the lid wafer 6 to the device wafer 7. Accordingly, a large capacitive coupling may occur, with commensurately large losses especially at high frequencies. Although the device shown in FIG. 1 may be smaller and lower cost than a TO-can packaging with ceramic carrier, the performance of the device may suffer from the aforementioned capacitive coupling, especially at higher frequencies.

[0031] FIG. 2 shows a first embodiment of the systems and methods disclosed here. In FIG. 2, there may be a laser light source 10 which produces a beam of light which may be shaped by a ball lens 20 and then through Faraday rotator 30. The beam of light then impinges on a turning surface 50 which redirects the light in a direction normal to the substrate, shown upward in FIG. 2. The light may pass through the lid substrate 60 which may encapsulate the aforementioned devices disposed on the device substrate 70. In FIG. 2, the turning surface is a turning mirror 50, which is a discrete structure, encapsulated in the device cavity along with the other components.

[0032] Suitable materials for the device substrate 70 and lid substrate 60 may be a metal or semiconductor such as silicon, or a ceramic or glass. The device cavity 65 may be etched into the lid wafer 60 using, for example, deep reactive ion etching (DRIE). The depth of the device cavity may be several hundred microns and have sufficient lateral extent to easily cover the components shown in FIGS. 2-8. Accordingly, the aforementioned components, including turning mirror 50, rotator 30, lens 20 and light source 10 may be disposed in the device cavity 65, such that the device cavity 65 encloses and encompasses the optical apparatus 110.

[0033] The laser 10 may be a light emitting laser diode for example, that can be driven by power and signal lines which are delivered to the laser 10 by one or more through silicon vias (TSVs) 40. These vias 40 are formed through the thickness of the device wafer 70. A number of references describe methods for making such through wafer vias 40. In the embodiment shown in FIG. 2, a discrete turning mirror 50 directs the beam of light from the laser 10, ball lens 20 and Faraday rotator 30 to a direction normal to the substrates. The beam of light may exit through the lid substrate 60.

[0034] This embodiment may make use of, for example, a single mode, distributed feedback (DFB) edge-emitting laser located within the device cavity, and thereby protected from the environment and moisture by a substantially hermetic seal. The single mode, edge emitting diode may be capable of higher data rates than a multimode vertical cavity surface emitting lasers (VCSELs), such that this embodiment may have both performance and cost advantages. The DFB laser may be modulated directly by a signal or waveform fed to the DFB laser through the through silicon via, or it may be driven by a direct current (DC) electrical signal applied to the TSV. However, it should be understood that the light source 10 may be at least one of a light emitting diode, a laser diode, an edge emitting laser diode, a laser diode, and a vertical cavity surface emitting laser. The electrical access to the TSV 40 may be provided by a bonding pad 80, to which macroscopic electrical connections may be made. In the embodiments shown in FIG. 2, because the light is emitted through the lid substrate and thus on the obverse side compared to the electrical connections, this embodiment may be particularly convenient for coupling to a printed circuit board or thin film circuit.

[0035] FIG. 3 shows another embodiment of the MEMS silicon optical apparatus. This second embodiment is similar to that shown in FIG. 2, except that in this embodiment, there is also a driver 15 that drives the laser 10 with a particular pattern or modulation that may represent data to be communicated over the optical link Like the previous embodiment, there is once again a laser light source 10, which produces a beam of light which may be shaped by a ball lens 20, and then rotated by a Faraday rotator 30. The beam of light then impinges on a turning surface 50 which redirects the light in a direction normal to the substrate, shown as upward in FIG. 4. The light may pass through the lid substrate 60 which encapsulates the aforementioned devices disposed on the device substrate 70. In FIG. 3, the turning surface is a turning mirror 50, which is a discrete structure, encapsulated in the device cavity along with the other components. As in the previous embodiment, the laser is driven by through substrate vias 40, which may improve the high frequency characteristics of the device. The electrical access to the TSV may be provided by a bonding pad 80, to which macroscopic electrical connections may be made. In the embodiments shown in FIG. 3, because the light is emitted through the lid substrate and thus on the obverse side compared to the electrical connections, this embodiment may be particularly convenient for coupling to a printed circuit board or thin film circuit. In the embodiment shown in FIG. 3, the TSVs may conduct a direct current (DC) signal to the driver 15, which then modulates the signal to encode information thereon. Accordingly, this embodiment may include the power driver inside the substantially hermetic package, and the close proximity of the compact device cavity provides for reduced power consumption. Therefore, the microfabricated optical apparatus may further comprise a device which modulates at least one of a frequency and an amplitude, to encode the optical radiation emitted from the light source with an information signal.

[0036] Otherwise, the embodiment shown in FIG. 3 may be similar to that shown in FIG. 2, and the turning mirror 50 may direct the optical radiation to exit the device cavity through a roof of the lid wafer, in a substantially parallel direction relative to the through silicon via.

[0037] FIG. 4 shows a third embodiment, wherein the turning mirror 50 directs the beam of light downward through the device substrate 70 rather than upward through the lid wafer 60. As in the previous embodiments, the laser may be driven by through substrate vias 40, which may improve the high frequency characteristics of the device. The output of this embodiment may be generally downward on the same side of the device as the electrical connections are made. Accordingly, in contrast to the embodiment shown in FIGS. 2 and 3, the optical apparatus in FIG. 4 has a turning mirror 50 which may bend the optical radiation to exit the device cavity through the device substrate 70, in a substantially parallel direction relative to the through silicon via.

[0038] FIG. 5 shows a fourth embodiment of the MEMS silicon optical apparatus, wherein the turning surface 50 is formed by a reflective surface on the lid wafer. This surface may be formed by anisotropic etching, followed by the deposition of a reflective coating on the lid wafer 60 surface. The reflective surface may be a thin film of gold (Au) or silver (Ag) or it may be a multilayer film with layer thicknesses designed to enhance reflectivity of the particular wavelength.

[0039] As in the previous embodiments, there is once again a laser light source 10, which produces a beam of light which may be shaped by a ball lens 20, and then rotated by a Faraday rotator 30. The beam of light then impinges on a turning surface 50 which redirects the light through the substrate, shown as generally downward in FIG. 5. The light may pass through the device substrate 70 on which the aforementioned devices are fabricated, in a non-normal (with respect to the substrate). As in the previous embodiments, the laser may be driven by through substrate vias 40, which may improve the high frequency characteristics of the device. The device may have the advantage of simpler fabrication. Accordingly, in some embodiments, the microfabricated optical apparatus may generate optical radiation which exits the device cavity 65 through a sidewall of the device cavity 65 in the lid wafer 60, at an angle with respect to the through silicon via. In this case, the turning surface may be a reflective film deposited on a sidewall of the device cavity, wherein the sidewall is inclined with respect to a surface of the substrate by about 50 to 60 degrees. The turning surface may be a reflective film deposited on an inclined surface of an optical element located within the device cavity.

[0040] FIG. 6 shows a fifth embodiment of the MEMS silicon optical apparatus, wherein a laser 10 generates a beam of light which is redirected upward by turning mirror 50. This turning mirror 50 directs the light upward through the lid substrate 60. A feature lens 20, may be formed on lid substrate 60 which can shape the beam of light as it passes therethrough. This embodiment is shown lacking some of the components described previously with other embodiments, such as the ball lens, Faraday rotator or isolator, and driver. It should be understood that these additional components may optionally be supplied with this embodiment as well. In FIG. 6, a horizontal line at the base of the lens 20 may suggest that lens 20 is a separate, distinct element. It should be understood that this horizontal line may be an artifact of the rendering of the illustration, and that lens 20 may be formed from a monolithic piece of silicon as described below.

[0041] The feature lens 20 may be formed using grey scale lithography, which makes use of a thick photoresist. Thick resists means, that the resist film thickness is much higher than the penetration depth of the exposure light. For standard positive resists and standard exposure wavelengths (g-, h-, i-line), this means a thickness of >5 m. (Of course, if small wavelengths with a very low penetration depth such as 310 nm are used, even a 1 m resist film will be thick in this context). Under these conditions, the resist film cannot be completely exposed towards the substrate. However, the resist may be bleached in the process as follows: In the beginning of the exposure, light only penetrates the upper 1-2 m of the resist film. This part of the resist film bleaches, so with the exposure going on, light will be able to penetrate the first 2-3 m of the film, and so on. As a consequence, the exposed (and developable) resist film thickness goes approx. linear with the exposure dose. The transition exposed/unexposed is sufficiently sharp for reproducible greyscale lithography applications.

[0042] When the grayscale exposed resist is used in an etching process such as the one used to make lens 20, the thin areas of the grayscale resist are removed early on, leading to relatively deeply etched features. The thicker areas of resist persist through the etching step, leading to shallowly etch features. Accordingly, the dome-shaped lens 20 is produced by having thin portions of the grayscale resist cover the horizontal surface of the substrate, and the thickest areas over the top of the dome of the lens 20

[0043] Grayscale lithography may be used to form a lens 20 on either the outer surface or the inner surface of the roof of the device cavity lid substrate. A lens 20 is shown on the outer surface in FIG. 7. Accordingly, the microfabricated optical apparatus may include a beam shaping element which is a lens formed in a roof of the device cavity.

[0044] As in the previous embodiments, there is once again a laser light source 10, which produces a beam of light which may be shaped by a ball lens 20, and then rotated by a Faraday rotator 30. The beam of light then impinges on a turning surface 50 which redirects the light in a direction normal to the substrate, shown as upward in FIG. 6 and downward in FIG. 4. The light may pass through the lid substrate 60 which may encapsulate the devices disposed on device substrate 60. As in the previous embodiments, the laser may be driven by through substrate vias 40, which may improve the high frequency characteristics of the device. The embodiment shown in FIG. 6 is shown lacking some of the components described previously with other embodiments, such as the ball lens, Faraday rotator or isolator, and driver. It should be understood that these additional components may optionally be supplied with this embodiment as well. The lens 20 may serve to shape, focus or collimate the light emitted from light source 10 as driven through the through silicon via (TSV).

[0045] FIG. 7 shows a sixth embodiment of the MEMS optical apparatus, wherein a plurality of lasers 10 each generate a beam of light which is redirected by turning mirrors 50. These turning mirrors 50 may direct the light in the same or different directions as the other light sources. One or more feature lenses 20, may be formed on lid substrate 60 which can shape the beams of light as they pass through. This embodiment is shown lacking some of the components described previously with other embodiments, such as the ball lens, Faraday rotator or isolator, and driver. It should be understood that these additional components may optionally be supplied with this embodiment as well. As shown in FIG. 7, the methods described here may be capable of manufacturing microfabricated optical apparatuses, wherein a plurality of light sources may be disposed in a single, compact, device cavity, along with the associated components.

[0046] The through silicon vias (TSVs) 40 which are shown in each of FIGS. 2-7 may be made by a number of techniques. In one approach, blind via holes are etched into the front side of a silicon substrate, but not extending through the thickness, such that material remains on the backside of the substrate. An insulating layer, for example, silicon dioxide SiO.sub.2 may then be grown on the bare silicon walls within the hole. A plating seed layer may then be deposited conformally in the hole. A conductive material such as copper, may then be plated into the hole. Finally, the remaining material may be removed from the backside of the substrate to expose the copper by, for example, grinding. The conductive copper may thereby extend through the thickness of the substrate 70. Additional details as to this method of making through silicon vias may be found in co-owned U.S. Pat. No. 7,233,048, which is incorporated by reference in its entirety.

[0047] Other methods may be used to form the vias, and some may be more appropriate for some substrate materials than others. These alternative methods may be found in, for example, U.S. patent application Ser. No. 11/482,944, U.S. Pat. No. 8,343791, U.S. patent application Ser. No. 14/499,287 and U.S. patent application Ser. No. 13/987,871. Each of these documents in incorporated by reference in their entireties, and each is owned by the owner of the instant invention.

[0048] The other optical components may be obtained as discrete devices, and disposed on the fabrication substrate by pick and place machines, similar to those used in printed circuit board manufacture to place components. These discrete optical elements may be held in place by epoxy or glue. The light source 10 may require a conductive bonding material to maintain conductivity with the through silicon via. This conductive bonding material may be, for example, a relatively low temperature gold/tin alloy bond.

[0049] As mentioned previously, the lid substrate 60 may have a device cavity 65 etched therein using, for example, deep reactive ion etching (DRIE) or anisotropic etching. Anisotropic etching tends to form sidewalls with a 56 degree slope with respect to vertical, whereas DRIE tends to make very sharp, very vertical features. Anisotropic etching may be used on the embodiment shown in FIG. 5, whereas DRIE may be used in the embodiments shown in FIGS. 2, 3, 4, 6 and 7. The 56 degree sidewall angle may be convenient for fabricating a reflective surface in order to direct the radiation out of the cavity.

[0050] After fabrication of the lid substrate 60 and placement of the optical elements within the perimeter of the device cavity, the lid substrate 60 may be bonded to the silicon device substrate 70. The bonding material may be, for example, a low temperature metal alloy bond such as gold/indium, which is formed at about 156 centigrade. Additional details as to methods for bonding with a gold and indium alloy may be found in U.S. Pat. No. 7,569,926, incorporated by reference in its entirety.

[0051] The embodiments illustrated in FIGS. 2-7 and described above have a number of advantages from a manufacturing perspective. They may be tested in a manufacturing environment with a conventional wafer probe to cull damaged or nonfunctional die. The design is capable of very high yield in a microfabrication production environment. They each allow integration of multiple lasers in a single device cavity, as was illustrated in FIG. 7.

[0052] More generally, a method for fabricating an optical apparatus on a substrate, may include forming a device cavity in a lid wafer, forming a through silicon via through the substrate, disposing a light source driven by a waveform which generates optical radiation on the substrate, and coupling the light source electrically to the through silicon via, disposing a beam shaping element on the substrate, disposing a turning surface which redirects the beam of light, and bonding the substrate to the lid wafer to encapsulate the optical apparatus in a substantially hermetic device cavity.

[0053] The method may further include etching a blind trench into a front side of the substrate leaving residual substrate material, coating the trench with an insulating material, depositing a conductive material in the blind trench, and removing the residual substrate material from a backside of the substrate to form the via. Even further, the method may include bonding the substrate to the lid wafer with a low temperature metal alloy bond.

[0054] The method may also include forming the device cavity with anisotropic etching, leaving inclined sidewalls in the device cavity inclined at angles of about 50 to 60 degrees with respect to a surface of the lid wafer, and depositing a reflective surface onto at least one inclined sidewall of the device cavity.

[0055] Finally, the method may include forming a lens in a roof of the device cavity, on an inside or an outside surface.

[0056] FIG. 8 is a simplified plan view of a fabrication substrate during processing in a manufacturing environment. As was described previously, the manufacturing method may be capable of fabricating a large number of like devices on a single fabrication substrate 100. These devices may each be microfabricated optical apparatuses 200. This fabrication substrate may be bonded to a lid substrate (not shown) with cavities and perhaps other structures previously formed therein, and registered with the optical apparatuses 200, to form a two-substrate assembly. The individual devices may they be singulated by sawing, dicing or grinding.

[0057] While various details have been described in conjunction with the exemplary implementations outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent upon reviewing the foregoing disclosure. Accordingly, the exemplary implementations set forth above, are intended to be illustrative, not limiting.