# Patent

## Title
Method for fabricating a device for propagating light

## Abstract

A method of forming a device for propagating light includes providing a substrate having a semiconductor material; placing an insulating layer on the substrate; providing a recess reaching through the insulating layer and into the substrate; filling the recess at least partially with a filler material; and arranging a waveguide in or on the filler material.

## Background
FIELD OF THE INVENTION
The invention relates to a device for propagating light and a method for fabricating a device for propagating light.
BACKGROUND
Photonic devices comprising a semiconductor are widely used in modern telecommunication systems. Typically the light is transmitted in optical fibers. Therefore, the light has to be coupled from a first photonic device into an optical fiber. After transmitting the light in the optical fiber the light has to be coupled back to a second photonic device. Therefore, coupling light from and to an optical fiber is an important aspect in the telecommunication field.
US 2013/0181233 A1 discloses a silicon photonics wafer that includes an active silicon photonics layer, a thin buried oxide layer, and a silicon substrate is received. The thin buried oxide layer is located between the active silicon photonics layer and the silicon substrate. An electrical CMOS wafer that includes an active electrical layer is also received. The active silicon photonics layer of the silicon photonics wafer is flip chip bonded to the active electrical layer of the electrical CMOS wafer. The silicon substrate is removed exposing a backside surface of the thin buried oxide layer. A low-optical refractive index backing wafer is added to the exposed backside surface of the thin buried oxide layer. The low-optical refractive index backing wafer is a glass substrate or silicon substrate wafer. The silicon substrate wafer includes a thick oxide layer that is attached to the thin buried oxide layer.
BRIEF SUMMARY OF THE INVENTION
A device for propagating light comprises: a substrate having a semiconductor material, an insulating layer, wherein the insulating layer is arranged on the substrate, a recess reaching through the insulating layer and into the substrate, wherein the recess is at least partially filled with a filler material, and a waveguide arranged in or on the filler material.
Further, a method for fabricating a device comprises: providing a substrate having a semiconductor material, placing an insulating layer on the substrate, providing a recess reaching through the insulating layer and into the substrate, filling the recess at least partially with a filler material, and arranging a waveguide in or on the filler material.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a perspective view of a device for propagating light;
FIG. 2 shows a cross-sectional view of the device for propagating light of FIG. 1;
FIG. 3 shows a cross-sectional view of a further device for propagating light;
FIG. 4 shows a cross-sectional view of a further device for propagating light;
FIG. 5 shows the device of FIG. 4 comprising a further waveguide;
FIG. 6 shows a top view of FIG. 5 with a tapered further waveguide;
FIG. 7 shows a cross-sectional view of a further device for propagating light;
FIG. 8 shows a cross-sectional view of a further device for propagating light coupled to a further waveguide; and
FIG. 9A to 9I show a method for fabricating a device for propagating light.



Similar or functionally similar elements in the figures have been allocated the same reference signs if not otherwise indicated.
DETAILED DESCRIPTION OF THE EMBODIMENTS
FIG. 1 shows a perspective view of a device 1 for propagating light. The device 1 comprises a substrate 2, an insulating layer 3, a recess 4 and a waveguide 5. The substrate 2 has a semiconductor material 16. The insulating layer 3 is arranged on the substrate 2. As can be seen in FIG. 1 the recess 4 reaches through the insulating layer 3 and into the substrate 2, wherein the recess 4 is filled with a filler material 6. The waveguide 5 is arranged on the filler material 6. For example, the substrate's semiconductor material 16 is oxidized such that the recess 4 filled with the filler material 6, e.g. silicon dioxide, is obtained. The boundary of the recess 4 in the substrate 2 can be defined as the interface 15 between the semiconductor material 16 and the filler material 6.
Further, the recess 4 has a recess bottom 7. A distance 8 between the waveguide 5 and the recess bottom 7 is larger than a thickness 9 of the insulating layer 3. The waveguide 5 is used for propagating light along the waveguide 5.
FIG. 2 shows a cross-sectional view of the device 1 along line II-II of FIG. 1. As can be seen the insulating layer 3 is arranged on the substrate 2. The recess 4 reaches through the insulating layer 3 and into the substrate 2. The light travelling through the waveguide 5 comprises a light mode 10. Part of the light intensity and light mode 10 is within the waveguide 5. However, the light mode will also have a certain intensity outside the waveguide 5. Thereby, the intensity decreases with increasing distance from the waveguide 5. The dotted line illustrates that the light mode 10 has a certain intensity outside the waveguide 5. However, there is no sharp limit or boundary where the light mode 10 does not have any more any intensity. Nearly all intensity of the light mode 10 is within radius 11 of the light mode 10. The radius 11 of the light mode 10 is less than half of the width 12 of the recess 4. Furthermore, the light mode 10 does not reach into the substrate 2.
As can be seen in FIG. 2 the radius 11 of the light mode 10 is larger than the thickness of the insulation layer. If the waveguide 5 would be arranged on the insulation layer 3 in an area far away from the recess 4 than the light mode 10 would reach into the substrate 2. However, because of the recess 4 the light mode 10 does not reach into the substrate 2. Therefore, an attenuation of the light travelling along the waveguide 5 because of the substrate 2 can be avoided. The light mode 10 does not feel the substrate 2 and the absorptive properties of the substrate 2.
The light mode 10 can be completely in the waveguide 5, the filler material 6 and the area 13 above the waveguide 5 and the filler material 6. Therefore, it is important that the filler material does not absorb light or at least does only absorb a very small amount of the light, most preferably the absorption is lower than the absorption of the substrate 2. Because of that the filler material is a non-absorbing material, wherein non-absorbing has the meaning not light absorbing.
The filler material 6 can have a lower index of refraction than the substrate 2. Further, the index of refraction of the filler material 6 can be lower than the index of refraction of the material of the waveguide 5. This helps to confine more light of the light mode 10 in the waveguide 5. As a result this means a lower loss of light due to absorption of light outside the waveguide 5.
The recess 4 can be formed such that a distance 14 between the waveguide 5 and an interface 15 between the semiconductor material 16 and the filler material 6 is larger than a predetermined distance. The predetermined distance can be calculated, for example, when the device 1 is designed. Further, the predetermined distance is the distance which is needed to keep the light mode 10 within the area of the filler material 6. Further, the predetermined distance can be larger than the radius 11 of the light mode 10.
As can be seen in FIG. 2 the recess 4 comprises a first side wall 17 and a second side wall 18. The waveguide 5 can be arranged such that a distance 19 between the waveguide 5 and the first side wall 17 equals a distance 20 between the waveguide 5 and the second side wall 18.
The insulating layer 3 can be a buried silicon dioxide layer. The filler material 6 and the insulating layer 3 can comprise the same material. This material can be silicon dioxide or other materials having a lower index of refraction than the substrate 2. The substrate 2 can comprise silicon. Using these materials allows fabricating the device 1 using the standard complementary metal-oxide-semiconductor technology (CMOS technology).
The waveguide 5 can comprise silicon, polysilicon or a polymer. In an alternative, the waveguide can consist of one of the group of silicon, polysilicon or a polymer. Furthermore, the waveguide 5 can comprise a cladding 52. The cladding 52 is optional and is therefore indicated with a dashed line in FIG. 2. As can be seen in FIG. 2 the cladding is on the top surface 53 and on the side surfaces 54 of the waveguide 5. In an alternative, the cladding 53 is only on the top surface 53 of the waveguide 5. In a further alternative, the waveguide 5 is surrounded by a cladding 52.
The locally increased thickness enables a larger radius 11 of the light mode 10. If the filler material 6 and the material of the insulating layer 3 are the same material than in principle the thickness of the buried oxide layer is locally increased.
FIG. 3 shows a cross-sectional view of a further device 1 for propagating light. In contrast to the device 1 shown in FIG. 2 in the device 1 shown in FIG. 3 the waveguide 5 is arranged within the filler material 6. The recess 4 is partly filled with the filler material 6. The recess 4 is formed such that a distance 14 between the waveguide 5 and the interface 15 between the semiconductor material 16 and the filler material 6 is larger than the predetermined distance. Thereby, the predetermined distance is the distance which is necessary to keep the light mode 10 within the area of the filler material 6.
Preferably, the waveguide 5 is arranged such that the distance 19 between the waveguide 5 and the first side wall 17 equals the distance 20 between the waveguide 5 and the second side wall 18. However, arranging the waveguide 5 in the center between the first side wall 17 and the second side wall 18 is not necessary.
FIG. 4 shows a cross-sectional view of a further device 1 for propagating light. In contrast to the device 1 depicted in FIG. 3 the device 1 depicted in FIG. 4 shows a recess 4 which has a tapered form 21 in the substrate 2 in a direction from the waveguide 5 towards the substrate 2.
The recess 4 has straight sidewalls 22 in the insulating layer 3. Further, in the substrate 2 the recess 4 has tapered sidewalls 23. The filler material 6 can be a fast grown oxide, which is thermally grown in a dry or wet atmosphere. Therefore, because of the way of depositing the filler material 6, the filler material 6 can provide tapered sidewalls 24 in the insulating layer 3. As can be seen in FIG. 4 the waveguide 5 is arranged on the filler material 6. The remaining areas 25 between filler material 6, insulating layer 3 and waveguide 5 can be filled with the same material as is also used for the insulating layer 3. As mentioned before also the filler material 6 and the material of the insulating layer 3 can be the same.
The tapered form 21 in the substrate 2 is well suited to fit the shape of the light mode 10. Since for the filler material 6, the material of the areas 25 and the material of the insulating layer 3 the same material can be used it is no problem if the light mode 10 reaches into the insulating layer 3 or the areas 25.
FIG. 5 shows the device 1 of FIG. 4 comprising a further waveguide 26. The further waveguide 26 is arranged above the waveguide 5. When the waveguide 5 and the further waveguide 26 are close to each other adiabatic coupling of light from one waveguide 5, 26 to the other waveguide 26, 5 is possible. In an alternative, the waveguide 5 and the further waveguide 26 are in contact with each other. In a further alternative, there may also be a gap between the waveguide 26 (which can comprise a polymer) and the waveguide 5 (which can comprise silicon). This gap is called “bondline”.
In an alternative, the further waveguide 26 can be arranged below the waveguide 5. In this case the waveguide 5 is arranged as depicted in FIG. 2. Further, in this case the further waveguide 26 is then arranged in the filler material 6.
FIG. 6 shows a top view of FIG. 5. However, in contrast to FIG. 5 the waveguide 5 of FIG. 6 comprises a tapered section 27. Since the waveguide 5 can not be seen from the top view because of the further waveguide 26, the waveguide 5 is depicted with a dashed line. The waveguide 5 extends in a longitudinal direction 28. In the area where the light adiabatically couples from one waveguide 5, 26 to the other waveguide 26, 5 the waveguides 5, 26 extend in the same direction. Therefore, the further waveguide 26 also extends in the longitudinal direction 28. FIG. 6 shows a situation where light is adiabatically coupled from the waveguide 5 to the further waveguide 26. When the light travels along the waveguide 5 it finally arrives at the tapered section 27. In the tapered section 27 the light mode 10 can't travel as before in the waveguide 5. Therefore, the light will couple to the further waveguide 26.
In an alternative, light is coupled from the further waveguide 26 to the waveguide 5. In this case the further waveguide 26 can comprise a tapered section.
In an alternative, both waveguides 5, 26 may contain tapered sections 27.
FIG. 7 shows a cross-sectional view of a further device 1 for propagating light. The insulating layer 3 comprises front-end electronic components 34. As an example a field-effect transistor (FET) 29 is depicted. The front-end-of-line (FEOL) is the first portion of integrated circuit (IC) fabrication where the individual devices (transistors, capacitors, resistors, etc.) are patterned in the semiconductor. The insulating layer 3 can further comprise an additional waveguide 30.
The device 1 can further comprise a passivation layer 31 on top of the insulating layer 3. The passivation layer 31 is also called isolation layer. The passivation layer can comprise silicon nitride. The passivation layer 31 is used to shield the front-end electronic components 34 from the back-end-of-line wiring and contaminants. The back-end-of-line (BEOL) is the second portion of integrated circuit (IC) fabrication where the individual devices (transistors, capacitors, resistors, etc.) get interconnected with wiring on the wafer.
The device 1 can further comprise back-end electronic components 32 which are arranged above the passivation layer 31. Such back-end electronic components are for example structured metal layers. Electrical contacts 33 are used to contact the front-end electronic components 34 with the back-end electronic components 32.
Furthermore, the back-end electronic components 32 can comprise a first interlayer dielectric (ILD1). Between the structured metal layers dielectric layers can be arranged to shield different structured metal layers from each other.
FIG. 8 shows a cross-sectional view of a further device 1 for propagating light. The device 1 is coupled to a further waveguide 26. In contrast to, for example FIG. 7, the propagation direction of the light is from left to right or from right to left depending if the light is coupled from the further waveguide 26 to the waveguide 5 or vice versa.
The further waveguide 26 can comprise a waveguide core 35 and a cladding 36. The waveguide core 35 can comprise polysilicon, silicon or a polymer. The waveguide core 35 is in direct contact with the waveguide 5 in the area where the light couples between the two waveguides 5, 26. As can be seen in FIG. 8 the filler material 6 is above the waveguide 5 and above the waveguide 5 is the substrate 2. The device 1 is supported by a carrier 37. The back-end electronic components 32 are in contact with the carrier 37 by coupling structures 38, e.g. C4 solder bumps. The coupling structures 38 can provide mechanical stability and electric contacts.
FIG. 9A to 9I show a method for fabricating a device 1 for propagating light.
FIG. 9A shows a step S1 of the method. First a semiconductor on insulator wafer (SOI-wafer) is provided. The SOI-wafer comprises a silicon layer 39, a silicon dioxide layer 40 and a top silicon layer 41 on top of the silicon dioxide layer 40. On the top silicon layer 41 a further silicon dioxide layer 42 is grown. Afterwards a silicon nitride layer 43 is grown on the silicon dioxide layer 42.
FIG. 9B shows a step S2 of the method. A photoresist 44 is deposited on the silicon nitride layer 43 and lithographically structured. Trenches 45 are etched with the silicon dioxide layer 40 as stop layer.
FIG. 9C shows a step S3 of the method. The photoresist 44 is stripped away. The trenches 45 are filled using the shallow-trench isolation (STI) electrical isolation scheme. The STI fill 46 consists of silicon dioxide deposited using a high-density plasma (HDP).
FIG. 9D shows a step S4 of the method. A chemical-mechanical polishing is performed to provide a planar surface 47.
FIG. 9E shows how a body contact can be realized. That means how a contact to the silicon layer 39 is provided. A photoresist 44 is deposited on the planar surface 47 and lithographically structured. Then the via 48 is etched with the silicon layer 39 as the stop layer.
FIG. 9F shows a step S5 of the method which is carried out analog to the realization of a body contact as depicted in FIG. 9E. However, in this case a wider via 49 is etched. The width 50 of the wider via is between 1 to 100 μm, more preferably between 5 and 20 μm.
FIG. 9G shows a step S6 of the method. A part of the silicon layer 39 is removed or transformed to provide the recess 4. Afterwards the recess 4 is at least partly filled with the filler material 6. The filler material 6 can be silicon dioxide. Preferably, the filler material 6 is thermally grown in a wet atmosphere. In an alternative, the filler material 6 may be grown through a dry atmosphere at elevated temperatures.
FIG. 9H shows a step S7 of the method. The photoresist 44 is stripped away. Further, the areas 25 between the filler material 6 and the silicon dioxide layer 40 are filled with silicon dioxide. Afterwards a chemical-mechanical polishing is performed to provide a planar surface 47.
FIG. 9I shows a step S8 of the method. Next the FEOL processing can be performed. After the FEOL processing no other processing steps need to be adjusted or introduced. A passivation layer 31 is then deposited on the silicon dioxide layer 40 which is the insulating layer 3. Further, the BEOL processing is performed. As shown in FIG. 9I an ILD1 51 comprising silicon dioxide is deposited.
Afterwards to finish the fabrication of the device 1 the ILD1 51 has to be removed in the area over the recess 4 and the waveguide 5 has to be arranged on top of the filler material 6.
More generally, while the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.
REFERENCE SIGNS


    1 device
    2 substrate
    3 insulating layer
    4 recess
    5 waveguide
    6 filler material
    7 recess bottom
    8 distance between waveguide and recess bottom
    9 thickness of the insulating layer
    10 light mode
    11 radius of the light mode
    12 width of the recess
    13 area
    14 distance between waveguide and interface
    15 interface between semiconductor material and filler material
    16 semiconductor material
    17 first side wall
    18 second side wall
    19 distance between waveguide and first side wall
    20 distance between waveguide and second side wall
    21 tapered form
    22 straight sidewall
    23 tapered sidewall in the substrate
    24 tapered sidewall in the insulating layer
    25 area
    26 further waveguide
    27 tapered section
    28 longitudinal direction
    29 field-effect transistor
    30 additional waveguide
    31 passivation layer
    32 back-end electronic components
    33 electrical contacts
    34 front-end electronic components
    35 waveguide core
    36 cladding
    37 carrier
    38 coupling structure
    39 silicon layer
    40 silicon dioxide layer
    41 top silicon layer
    42 silicon dioxide layer
    43 silicon nitride layer
    44 photoresist
    45 trench
    46 shallow-trench isolation fill (STI fill)
    47 planar surface
    48 via
    49 wider via
    50 width
    51 interlayer dielectric 1 (ILD1)
    52 cladding
    53 top surface
    54 side surface

## Claims

1. A method for fabricating a device for propagating light, comprising:
providing a substrate having a semiconductor material;
placing an insulating layer on a surface of the substrate;
forming a recess to penetrate both of a portion of the insulating layer and a portion of the substrate;
filling the recess at least partially with a filler material; and
arranging a waveguide on the filler material,
wherein a bottom surface of the waveguide is coplanar with a top surface of the insulating layer, and
wherein the recess has a recess bottom, and a distance between the waveguide and the recess bottom is larger than a thickness of the insulating layer.

2. The method of claim 1, wherein providing a recess comprises:
etching the insulating layer to access the substrate; and
removing or transforming a part of the semiconductor material of the substrate.

3. The method of claim 2, wherein filling the recess comprises:
filling the recess at least partly with the filler material which has a lower index of refraction than the substrate; and
filling the etched insulating layer to form a planar surface of the insulating layer.

4. The method of claim 1, wherein the filler material has a lower index of refraction than a material of the waveguide.

5. The method of claim 1, wherein the recess is formed such that a distance between the waveguide and an interface between the semiconductor material and the filler material is larger than a predetermined distance.

6. The method of claim 5, wherein the predetermined distance is larger than a radius of a light mode of the waveguide.

7. The method of claim 1, wherein the filler material has a lower index of refraction than the substrate.

8. The method of claim 1, further comprising: tapering the formed recess such that the recess has a tapered form in the substrate in a direction from the waveguide towards the substrate.

9. The method of claim 1, arranging a further waveguide below or above the waveguide for an adiabatic coupling of light between the two waveguides.

10. The method of claim 9, further comprising: extending the waveguide in a longitudinal direction and tapering the waveguide or tapering the further waveguide in the longitudinal direction.

11. The method of claim 1, further comprising: forming front-end electronic components at the insulating layer.

12. The method of claim 1, further comprising: forming a passivation layer on top of the insulating layer.

13. The method of claim 12, further comprising: forming back-end electronic components above the passivation layer.