Patent Application
20210286203
The present inventive concept relates to the field of fabrication of electro-optical devices. More particularly it relates to a method of fabricating an electro-optical modulator for modulating electromagnetic waves in a radiofrequency (RF) waveguide.
Both long-haul and short-distance network interconnects for conventional data networks and intra-/interchip data links continue to scale in complexity and bandwidth. As interconnect densities rise, the limitations of copper as an interconnect medium in terms of its loss, dispersion, crosstalk and fundamental speed becomes more eminent. Thus, the optical interconnect, with silicon photonics, with an optical medium of silicon, emerging as a leading approach because of its unique combination of low fabrication costs and performance enhancements. Electronic-photonic integration and compatibility with the world's most successful technology for producing electronics, CMOS, makes integrated photonic circuits more appealing.
One of the essential components of any communication link is the modulator. An optical modulator is a device that modulates a light beam propagating either in free space or in an optical waveguide. Based on the beam parameter these devices affect, they are categorized as either amplitude, phase or polarization modulators.
Applying an electric field to a material changes its real and imaginary refractive indices. A change in the real part of the refractive index with an applied electric field is known as electro-refraction effect and a change in the imaginary part of the refractive index is known as electro-absorption effect. Pockels effect, Kerr effect and the Franz-Keldysh effect are the physical phenomena which generate the refractive index change. Unfortunately, these effects are weak in pure silicon at the telecommunications wavelengths (1.3 to 1.55 μm). One of the alternative methods to achieve modulation in silicon is thermal modulation owing to the large thermo-optic coefficient of silicon. But this is too slow for the high frequencies required by modem telecommunications applications.
Lithium niobate (LN) electro-optical modulators are widely available as packaged commercial components from several suppliers, and there has been tremendous progress on large bandwidth, low power integrated lithium niobate modulators in recent years. All these interesting results are from different university research groups. The next phase would be mass production of these modulators using the foundry technologies. The problem with current state of the art designs is that none of them are fully compatible with conventional fabrication processes for producing electro-optical devices. Therefore, there is a need in the art for methods of fabricating electro-optical devices that are compatible with conventional mass-production processes for microelectromechanical systems (MEMS), i.e. compatible with standard processes and foundries for MEMS.
It is an object of the invention to at least partly overcome one or more limitations of the prior art. In particular, it is an object to provide a method of fabricating an electro-optical modulator for modulating electromagnetic waves in a radiofrequency (RF) waveguide.
It is a further object of the present invention to provide at least one method for large-scale, batch, or other foundry-level fabrication of an electro-optical device that are compatible with conventional mass-production processes for microelectromechanical systems (MEMS), i.e. compatible with standard processes and foundries for MEMS.
As used herein, the term vertical denotes a direction being parallel to a vertical geometrical axis extending perpendicular to the substrate, i.e. the SOI wafer. The terms “above”, “below”, “upper” and “lower” are thus used to refer to relative positions along the vertical axis. In addition, the term lateral or horizontal refers to the direction perpendicular to the vertical direction, i.e. to the direction parallel to the substrate surface.
Further, herein the abbreviation “RF” refers to radio-frequency (unless otherwise indicated).
As a first aspect of the invention, there is provided a method of fabricating an electro-optical device, comprising
The electro-optical device, for example, may be an electro-optical modulator. An electro-optical modulator is a device that can be used to control the power (amplitude), phase (delay), or polarization of an optical beam with an electrical signal. As an example, the electro-optical modulator may be configured such that the desired modulation is performed by changing optical parameters such as refractive index and absorption of the waveguide according to the modulating signal.
The electro-optic modulator may be a traveling wave modulator, in which an RF signal is used to modulate an optical signal.
At least one RF electrode is provided in the upper portion oxide layer of the SOI wafer, such in the silicon dioxide layer of the SOI wafer. Thus, the at least one RF electrode is provided closer to the upper surface of the oxide layer than to the lower surface.
Each of the RF electrodes may be a metallic strip. The metallic strip may function as an RF electrode. As an example, the electro-optical device may comprise several RF electrodes, such as at least three RF electrodes.
The RF electrodes may be spaced such that they together form an RF waveguide, such as a coplanar waveguide (CPW).
The SOI wafer is a layered silicon-insulator-silicon substrate commonly used in the fabrication of silicon semiconductor devices. The oxide layer of the SOI wafer may be the uppermost layer of the SOI wafer.
A first aspect of the present invention is based on the insight that fabricating the RF electrodes within the oxide layer of the SOI wafer minimizes the post processing needed after the foundry fabrication of the electro-optical device. In prior art devices, the RF electrodes are placed on top of the modulating layer, e.g. on top of a lithium niobate layer. In the method of the first aspect, the RF electrodes are fabricated in the oxide layer of the SOI wafer. After that, the modulating structure with its substrates is bonded to the SOI wafer using either direct or indirect bonding. Consequently, in the design of the present disclosure, the RF electrodes are inserted in e.g. the SiO2 layer of the SOI wafer, which is thus below the modulating structure. The method of the present disclosure provides a fabrication process that is completely foundry compatible except for the bonding of the second substrate on top of the SOI wafer, which may be a back end of the line process that is performed outside the foundry.
The SOI wafer may comprise at least two RF electrodes, such as at least three RF electrodes.
The RF electrode may for example be metal tubes or pipes through which electromagnetic waves are propagated in microwave and RF communications. The wave passing through the medium may thus be forced to follow the path determined by the physical structure of the guide. As an alternative, the RF electrode components may, under certain conditions, contain a solid or gaseous dielectric material.
The RF electrodes may be made from aluminium (Al), chromium (Cr), gold (Au), brass (CuZn), bronze (CuSn), copper (Cu), or silver (Ag), for example All RF electrodes of the electro-optical device may be of the same material or of different materials.
In embodiments of the first aspect, the RF electrode comprises at least one metal selected from the group comprising gold (Au), chromium (Cr) and aluminium (Al). As an example, an RF electrode may comprise a mix of materials, such as both chromium (Cr) and gold (Au).
The RF electrode may be arranged in the uppermost portion of the oxide layer. In embodiments of the first aspect, the at least one RF electrode is arranged within the silicon oxide layer such that it forms part of the upper surface of the silicon oxide layer of the SOI wafer.
Thus, the second substrate may be bonded directly or indirectly to the oxide layer of the SOI wafer, and the RF electrodes may form part of this uppermost layer of the SOI wafer provided in step a), described below.
The second substrate has a top structure of an RF modulating material. The top structure may be formed as the uppermost layer of the second substrate. However, there may also be a further layer above the RF modulating material of the second substrate, such as a layer that facilitates bonding to the SOI wafer.
As an example, the top structure may be a layer, such as a top layer, of an RF modulating material as is well understood in this art.
In embodiments of the first aspect, the RF modulating material is selected from the group comprising of lithium niobate and barium tatanate.
Lithium niobate, or LN (LiNbO3) is intrinsically a birefringent crystal that is widely used in electro optic devices. Barium tatanate, or BTO (BaTiO3) provides a strong electro-optical effect.
Thus, the electro-optical device may be a thin film lithium niobate traveling wave modulator.
However, the RF modulating material may also be PZT (Lead zirconate titanate).
As an example, the top structure may be a top functional layer of a 300-900 nm LN (LiBNO3) film. Such a film may be optionally doped with magnesium oxide (MgO).
In embodiments of the first aspect, the SOI wafer further comprises at least one optical waveguide. The optical waveguide may be of silicon (Si). In the fabricated electro-optical device, at least some optical waveguides and the RF electrodes may be located on the same side (underneath) the structure of a RF modulating material. The optical waveguides may for example be arranged at lateral positions that are in between the lateral positions of the RF electrodes. The optical waveguides may be part of a common optical waveguide structure.
In embodiments of the first aspect, step a) further comprises
The RF electrodes may be fabricated in a SOI wafer by etching trenches in the oxide layer of the SOI wafer at the location of the RF electrodes using standard silicon foundry processes, such as dry etch. The RF electrode may then be fabricated in the formed trenches.
Step (a1) may also comprise forming at least one optical waveguide on top of the silicon dioxide layer. Thus, the optical waveguides may be fabricated on the SOI wafer before etching and formation of the RF electrodes. This means that both optical waveguides and RF electrodes may be on the same side of the modulating material in the electro-optical device.
As a further example, step (a3) may comprise depositing a seed layer for the RF material in the trenches followed by electroplating the RF material; thereby filling the trenches with the RF material.
Consequently, the RF material may be grown by first depositing a seed layer, such as a gold (Au) layer, in the trenches before electroplating the RF material. This seed layer may for example be between 10-20 nm. It may be advantageous to use a seed layer if the RF material has low adhesion to the oxide layer of the SOI wafer. As an example, the RF material may be or comprise gold (Au), and it may then be useful to deposit a thin seed layer before electroplating the gold (Au) in the trenches.
Material that has been deposited outside the trenches may then be removed using CMP or similar processes.
In embodiments of the first aspect, the second substrate is bonded top side down to the SOI wafer in step (c).
The top structure of a RF modulating material may thus be bonded such that the top structure faces the SOI wafer.
Bonding of the second substrate on top of the SOI wafer may be performed using both indirect bonding and direct bonding.
Consequently, in embodiments of the first aspect, the bonding of the second substrate on top of the SOI wafer in step (c) is performed by indirect bonding.
In direct bonding may comprise first depositing an intermediate layer on the SOI wafer and/or second substrate, and then bonding the SOI wafer and the second substrate. As an example, the indirect bonding may be adhesive bonding, (or glue bonding) which may comprise depositing an organic or inorganic layer, such as SU-8 or benzocyclobutene (BCB), on one or both of the SOI wafer and the second substrate.
Thus, as an example, step (c) comprises depositing a polymer layer on top of the SOI wafer and bonding the second substrate top side down on top of the polymer. Such a polymer may be a benzocyclobutene (BCB) based polymer.
However, in embodiments of the first aspect, the bonding of the second substrate on top of the SOI wafer in step (c) is performed by direct bonding.
A direct bonding may lead to chemical bonds between the second substrate and the outer surface of the SOI wafer. Such a direct bonding may be realized by using a planarized silicon oxide layer between the SOI wafer and the second substrate.
As an example, the silicon oxide layer may be deposited on the SOI wafer before bonding to the second substrate.
The second substrate may be bonded to the SOI wafer such that portions of the at least one RF electrode is not covered by the second substrate. These uncovered portions may be used for applying an RF signal to the RF electrodes. As an alternative, the second substrate may be bonded such that it fully covers some or all of the RF electrodes, and contacts to the RF electrodes may be formed in a later processing step.
In embodiments of the first aspect, all method steps except the except for the bonding of the second substrate on top of the SOI wafer, are performed in the same foundry line process. Bonding of the second substrate to the SOI wafer may then be performed using a back end of the line process that is performed outside the foundry.
According to a second aspect of the present inventive concept, there is provided an electro-optical device comprising:
This second aspect may generally present the same or corresponding advantages as the first aspect. Effects and features of this aspect are largely analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with this aspect.
This aspect thus relates to an actual electro-optical device, such as an electro-optical modulator, which for example may be fabricated using the method of the first aspect discussed above. Thus, in embodiments, the electro-optical device is an electro-optical modulator.
The silicon layer may be the bottom layer of the device, and the silicon oxide layer may be arranged on top of the silicon layer.
In the device, the RF modulating material may be selected from the group comprising lithium niobate and barium tatanate.
Furthermore, the intermediate layer may be a polymer layer, such as a benzocyclobutene (BCB) based polymer layer. Such a layer may have been used when bonding a wafer comprising the RF modulating material to an SOI wafer comprising a silicon layer, a silicon oxide layer and at least one RF (radio frequency) electrode arranged within the upper portion of the silicon oxide layer.
However, as discussed in relation to the method above, the intermediate layer may be a silicon oxide layer, e.g. used in a direct bonding process.
In embodiments, the device further comprises at least one optical waveguide. Such an optical waveguide or waveguides may be arranged between the at least one RF electrode and the structure of a RF modulating material.
Hence, the electro-optical modulator may be an integrated travelling wave Mach-Zender modulator in which the RF electrodes are fabricated on the same layer as the optical waveguides, such that the RF electrodes and the optical waveguides are both arranged underneath the RF modulating structure.
Moreover, the RF electrode may comprise a material selected from the group comprising gold (Au), chromium (Cr) and aluminium (AI).
Further, the electro-optical device may comprise contacts arranged for applying an RF signal to the RF electrodes. The contacts may for example be arranged at portions of the RF electrodes that are not covered by the second substrate.
The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.
In the above disclosure the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.
Moreover, there are two optical waveguides 8 arranged on top of the silicon dioxide layer 5, but at different locations than the RF electrodes 6.
There is also a structure of a RF (radio frequency) modulating material 9, in the form of a layer of lithium niobate (LiNbO3, also abbreviated as “LN”) arranged in a layer over the silicon dioxide layer 7. This structure or layer 9 is arranged at a distance from the RF electrodes 6 such that it provides for modulating the electromagnetic waves propagating in the RF electrodes, e.g. by a change in the strength of the local electric field. Thus, the electro-optical device 1 may comprise control means for controlling the electric field at the position of the LN (LiBNO3) layer 9.
In the electro optic device of
As illustrated in
As illustrated in
Thereafter, as illustrated in
As illustrated in
Then, as illustrated in
During the electroplating process, trenches 13 are thus filled with a RF material, such as gold (Au) and or chromium (Cr), thereby forming the RF electrodes 6. Electro-plating is a standard process that uses an electric current to reduce dissolved metal cations so that they form a thin coherent metal coating on e.g. an electrode (or in this case within the trenches 13). Electro-plating process is in this example used to form thick RF electrodes of e.g. gold (Au) in the order of few (1-2) micrometres.
Oxide 15 and any RF material outside the trenches 13 are removed by standard dry or wet etching techniques. The formed structure is illustrated
Finally, only the active part of the SOI wafer 2 should be covered by an RF modulating material such as lithium niobate (LiBNO3, LN). The active parts include the parts where the RF electrodes 6 are positioned. For this purpose, a second substrate 3 in the form of a LN (LiBNO3) wafer is provided. Such a wafer 3 is schematically illustrated in
The whole second substrate 3, or pieces of it, may then be bonded to the SOI wafer 2 such that the top functional layer 9 is arranged vertically above the RF electrodes 6. Thus, the whole SOI wafer does not have to be covered with the second substrate 3. Further, not all parts of the RF electrodes 6 may be covered by the second substrate 3. Uncovered portions may be used for applying an RF signal to the RF electrodes 6, e.g. by forming contacts to such uncovered portions.
The second substrate 3 may be bonded top-side down to the fabricated SOI wafer 2, as illustrated in
However, the second substrate 3, i.e. the LN (LiBNO3) wafer, may be bonded by other means than using a BCB (benzo-cyclo-butene) layer to the first substrate 2. As an example, an SiO2 layer may be deposited on top of a first substrate 2 that has been manufactured as illustrated in
Consequently, the bonding of the second substrate 3 on top of the SOI wafer 2 may be performed by different methods, as summarized below:
As further seen in
The two ends of the RF electrodes 8 are not covered by the LN (LiBNOs) wafer 3, and they may be used for applying an RF signal. When an RF signal is applied over the electrodes 6, a phase shift may be induced for the electromagnetic wave passing through the waveguide arms 8b and 8c and when the two arms 8b and 8c are recombined, the phase difference between the two waves is converted to an amplitude modulation. Thus, unmodulated optical signal is fed into the optical waveguide 8 from one side 8a and modulated optical signal is extracted from the other end of the optical waveguide 8.
In the above, the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.
