WO2025131823A1

WO2025131823A1 - Glass circuit board and glass core based package with optical functionalities

Info

Publication number: WO2025131823A1
Application number: PCT/EP2024/085282
Authority: WO
Inventors: Jens Ulrich Thomas; Ulrich Fotheringham; Martin Letz
Original assignee: Schott AG
Current assignee: Schott AG
Priority date: 2023-12-21
Filing date: 2024-12-09
Publication date: 2025-06-26
Anticipated expiration: 2026-06-21

Abstract

The invention provides a package comprising a glass substrate and a semiconductor substrate, wherein the semiconductor package is at least in areas aligned to the glass substrate. The glass substrate can comprise a waveguide and can be adapted to provide a low thermal mismatch.

Description

Glass Circuit Board and Glass Core based Package with optical Functionalities

The subject of the invention is a glass board and/or a glass core based semiconductor package with semiconductor components where at least data transport is done using optical waveguides in the glass board. Using these in-plane waveguides, a board to chip optical signal transport is realized capable of a bandwidth of 100Gbit/s and more. The optical functionality can be combined with through glass vias (TGV’s) which are needed for electrical signal transport, for power supply and for thermal management of the whole package.

In the package also electrical short range chip-to-chip connections can be realized with a RDL (redistribution-line) distance of 2 pm and smaller. Thermal management can be improved by choosing glasses which are adjusted with respect to their thermal expansion to the metals (e.g. Cu) and polymers in use.

Currently the end of Moore’s law comes in reach, which manifests itself in the fact, that the speed, at which the density of structures on a single Si wafer increases, is slowing down. In parallel, the worldwide demand for highly efficient computer nodes driven by artificial intelligence (Al), large language models and other data-center driven applications is strongly increasing. Advanced packaging is the method of choice to handle the limited yield in high end chip manufacturing and to further increase the performance of semiconductor components. Advanced packaging means heterogeneous integration of different chiplets, which are produced on different semiconductor fabs together with passive components, power supply, voltage conversion or for telecommunication applications even with antenna and other analog RF (radio frequency) functionalities.

State of the Art Such heterogenous integration in advanced packaging has been already realized using silicon as an interposer material. Silicon is well established in the semiconductor industry and readily available. However, silicon has three major disadvantages:

It has (i) a relatively high dielectric constant which leads to large parasitic capacitances and long signal delay times.

It has (ii) a relatively high dielectric loss of tan(5)~0.01 and larger which needs to electric losses and most important

(iii) it is available in round wafers with a diameter of up to 300mm. The latter makes it economically difficult to manufacture large high-performance packages with a geometry of around 100mm x 100mm.

Recently a new material class has come into play. This is glass panel-based packaging. Glass simultaneously offers low dielectric constant, low dielectric losses down to around tan(b) ~ 0.001 (at 10GHz) and is available in panel format. A format which is being established right now is the size of 510mm x 515mm. In addition to the advantages over silicon, glass is transparent and offers the possibility to embed waveguides into the glass. Here different possibilities exist, most of them being based on photolithography.

With special suitable glass substrates, the negative pattern of a structured protective layer on top of the substrate can be transferred into the latter by ion exchange, see, e.g., DE3501898C2 and the literature cited there. It was also reported, that with SCHOTT glass D263Teco as substrate material and by exchanging sodium (Na+) ions in the glass for silver (Ag+) ions thus creating Ag+- doped waveguide cores in an undoped medium, with a refractive index difference of about 0.02. The patterning of the protective layer is done by photolithography.

According to the prior art, a further implementation possibility exists, which allows much higher contrasts in refractive index and therefore much smaller mode field diameters is to combine a glass substrate (typical refractive index 1 .5) with a high index core material such as silicon nitride (SiN, refractive index ca. 2.6). This can be done by coating a glass substrate with a SiN layer which later will be patterned by photolithography. After patterning, the glass substrate will be covered with a network of SiN lines, the later waveguide cores. The whole structure will then be coated with SiO2 to completely encapsulate the SiN cores with a lower index material. It can be noted that the glass substrate can be SiO2 which in return can be manufactured by oxidizing the surface of a silicon wafer. An analogous procedure starts structuring a glass substrate with reactive ion etching (RIE) of trenches and filling these trenches with high refractive index material like SiN. RIE waveguides are only possible in a few glasses.

It is also known to inscribe waveguides into glass substrates with ultra short pulsed lasers, as has been exemplarily described in US 7262144 B2.

Problem to be solved

Electric data transport at very high data rates, in the range of Gbit/s, is very expensive and underlies high damping. This is due to the fact, that Gbit/s data rates contain GHz frequency components. Here typical damping is in the order of some dB/mm for electrical data transport in e.g. microstrip lines. Optical data transport is by far more efficient. Here a normal waveguide, which has still a large potential for further optimization, shows a damping of order of -0.01 dB/mm. Replacing an electronic data transport by optical data transport already becomes preferable when the transport length of a few millimetres is exceeded.

Up to now, the coupling of glass substates with or without optical functionality to semiconductor devices is challenging. Therefore the invention aims towards a hybrid glass and semiconductor package, which can provide the desired electrical and/or optical properties and which is rational to manufacture.

Description of the invention In the present invention the problem of data transport is solved by using the glass core in the package to also transport data in optical waveguides which are written into the glass core of the package. A special challenge is the coupling of the optical signal into the converter, the photo detector and vice versa the coupling from the light source, e.g. a modulated laser diode into the optical waveguide.

Advantageously, for example Borosilicate glasses are used as interposer and/or for embedded waveguides. Embedding waveguides, Bragg gratings, refractive and reflective elements can be inscribed within the glass material with means of fs-laser writing.

Another advantageous embodiment comprises that the waveguide in introduced into the glass material by means of ion exchange.

In a general perspective, the invention contributes to the following aspects:

• Optical coupling from fiber to the photo detector and the electronic chip

• Optical routing through the glass core containing board

• Optical routing through the glass core of a package

One process to realize the optical waveguides could be a structured silicon oxynitride layer on top of glass package substrate. Oxinitrides can be structured using oxygen sputtering via chemical vapor deposition (CVD) and Ar plasma cleaning processes. The silicon oxynitride has a higher refractive index then the glass substrate, hence it is guiding the light in between its top surface and the glass/oxynitride interface. The sputtering process can be combined with a lithographic masking process, resulting 2 dimensionally confined ridge waveguides.

In case the waveguide is introduced into the glass substrate by ion exchange, there is an abundance of glasses suitable for chemical strengthening by ion exchange. They all have in common that they are supposed to have a high value of the so- called chemical expansion coefficient and thus build up considerable stress during the ion exchange, which is usually an exchange of sodium for potassium, in some cases also lithium for sodium or a combination of both. They also have in common that the thermal expansion coefficient is high.

Obviously, both issues are not relevant for the type of ion exchange relevant for the present invention. High stresses being generated during the ion exchange are rather counterproductive than desirable, as is a high thermal expansion coefficient. For a better fit to the semiconductor materials which shall be attached to the glass substrate, a low thermal expansion glass is preferable or at least a glass which is a compromise in thermal expansion between the semicon world (CTE~3.3 ppm/K) and the copper metallization or dielectrics (e.g. Ajinomoto buildup film (ABF) epoxy based dry-film dielectrics).

There are already several possibilities for optical data transport in packages. One possibility are polymer waveguides which are brought on top of the interposer or printed circuit board. Polymer waveguides have however relatively high loss and have extremely large temperature coefficients. Currently the optic to electronic signal conversion happens at the edge of the printed circuit board of a server. From there on all signal are transported electronically, which means they are transported with high losses, which need additional effort, energy and resource for further amplification and improvement of signal to noise ratio.

The invention foresees edge coupling in a substrate using waveguides. As well, the packages can be coupled to external optical cables. Especially, a 90° deviation to couple light from a waveguide into grating coupler elements is possible.

Furthermore, the combination of electrical through glass vias, short TGVs, with optical waveguides can be realized utilising the content of this invention.

Furthermore, reflective mirrors inside a glass substrate to couple optical signals into a photo detector or emitter. On a channel level, the overall transmission T efficiency is governed by the coupling losses in and out of the waveguide and the losses of the waveguide itself. We define Tin and Tout the overall coupling efficiencies in and out of the waveguide and Twg the transmission efficiency of the waveguide. The overall transmission efficiency is then the product T=TinT_WgTout.

Advantageously T>0.6, T>0.8, T>0.9, T>0.95.

Typically, the coupling efficiencies are governed by the Transmission of the end facet coating Tcoating at the preferred communication wavelength X and the overlap integral Tcoupling to be T(in/out)⁼Tcoating(in/out)*Tcoupling(in/out).

The transmission efficiency of a straight waveguides is

Twg = 1 0^A(Lw/1 0).

For bend waveguides Twgbend ⁼ (1 -bending losses) Twgstraight ^<= Twgstraight. The bending losses are a function of the bending radii, which are preferably >100 mm, >200 mm, >500 mm, >1 m.

The communication wavelength I is set by the optocoupler of the chip. Typically X = 1300 nm, 1550 nm or 530 nm or 900 nm.

The invention has the advantage that at least one, advantageously a combination of some or even all parameters can be achieved by the invention:

• Waveguide losses L_w: smaller than -0.5 dB/cm, more preferred -0.3, -0.1

• Mode field diameters: 5-15 pm for 1550 nm communicaiton wavelength, 2-10 for 533 nm communicaiton wavelength.

• (Multi-) Modality: V-Parameter 0.9, 1.4, (Single mode), 2-5 or 10 for MM

• Transversal refractive index profile: rectangular or gradient

• Minimum waveguide bend radius >500 mm

• Coupling integral T >0.5, 0.7, 0.9, 0.95, 0.98, 0.99 The coupling integral Tcoupi is defined by the following formula:

Here, F_r(x, y) is the function describing the receiving fiber complex amplitude, W(x, y) is the function describing the complex amplitude of the beam coupling into the fiber, and the ’ symbol represents complex conjugate.

For the waveguiding properties, the transversal refractive index profile of the modification line is relevant. For the following, we introduce the local coordinate vectors x and y, which are both perpendicular and centered to the waveguide center. The refractive index profile An(x,y) can be approximately rectangular: e.g. it can be described with An (x,y) = Anmax for |x| < w_x/2 and |y|< w_y/2 and An = 0 elsewhere.

Here w_x and w_y are the transversal widths of the waveguides, which are typically 0.2 pm to 100 pm, most preferred 0.5 to 20 pm, mostly preferred 1-15 pm, more mostly preferred 2-12pm.

The profile can also be more gradient like, e.g.

An (x,y) = Anmax - ( ai |x| + a2X² + a3|x|³ + a4X⁴ + ... + bi|y| + b2y² + b3|y|³ + b4y⁴ + ....), where ai and bi are constants greater or equal zero. This profile can also be altered along the waveguide. In this case we relate to the FWHM definition of the waveguide width along the local x and y axis.

The waveguides lines of this invention have low losses for light at the wavelength of 1550 nm, namely below -1 dB/cm, preferably -0.5 dB/cm, most preferred smaller than -0.3 dB/cm, mostly preferred below -0.1 dB/cm.

The V-Parameter is set by the respective waveguide widths w:

V=7i Wcore NA/X, where NA is the numerical aperture of the waveguide (in good approxation of a rectangular refractive index profile: NA = (An²max)^A(1/2). One can also determine the NA of the waveguide experimentally, its exiting light exits conelike, with a being the cone angle and NA=sin a. For single mode operation one adapts the transversal refractive index profile so that V<2.405. If one wants to encode the signal over more than one optical mode, V is chosen to be greater than 2.4. The total number of modes M can be approximated with M = 4V²/K²

In other embodiments, the package according to the invention comprises a compound optoboard with an ultrathin (< 200pm) flex-glass-layer for bendable waveguide layer, whereas the glass is advantageously hardened.

Advantageously, the CTE mismatch between the glass substrate and the semiconductor substrate is below 10 ppm/K, more advantageously <5 or <3 or <1 or <0.5 ppm/K. The CTE mismatch is measured typically at room temperature (25°C).

As well advantageously, an optical (telecom-) fiber can be glued in optoboardcavities.

Furthermore, a waveguide can be introduced onto or within the glass substrate applying a SiN-coating or RIE with subsequent SiN filling of trenches. Alternatively, femtosecond-laser inscription is possible as well or ion exchange

The invention can be summarized by the following sentences:

A package comprising a glass substrate and a semiconductor substrate, wherein the semiconductor package is at least in areas aligned to the glass substrate.

A package comprising a glass substrate and a semiconductor substrate, wherein the semiconductor package is at least in areas aligned to the glass substrate, where their relative alignment error Ax, Ay is smaller than 5 pm, 3 pm, 1 pm, 0.5 pm, 0.1 pm and/or their combined alignment error Ar² = Ax² + Ay² is smaller than 5 pm, 3 pm, 1 pm, 0.5 pm, 0.1 pm. A package comprising a glass substrate and a semiconductor substrate, where the aforementioned alignment errors remain in the aforementioned range, when the semiconductor package is heated from 20°C to 100°C.

The package according to the preceding sentence, wherein the glass substrate comprises a waveguide, which is coupled to an electronic or optoelectronic device or electric conductor within or attached to the semiconductor substrate.

The package according to at least one of the preceding sentences, wherein the glass substrate has an aligning surface, which is matched to an aligning surface of the semiconductor substrate.

The package according to the preceding sentence, wherein the aligning surface of the glass substrate comprises a recess in the glass substrate.

The package according to at least one of the preceding sentences, wherein the waveguide within the glass substrate is located on another plane than the semiconductor substrate; preferably the optical signal being transmittable within the waveguide is redirected to the plane of the semiconductor substrate by means of an optical redirection device, most preferably a prism, which is especially preferably located in a recess of the glass substrate.

The package according to at least one of the preceding sentences, wherein the waveguide within the glass substrate is located on another plane than the semiconductor substrate; preferably the optical signal being transmittable within the waveguide is redirected to the plane of the semiconductor substrate by means of an optical grating, most preferably a diffraction grating.

The package according to at least one of the preceding sentences, wherein the glass substrates contains TGVs, which preferably are fully or conformally filled with a metal to in order to at least partially serve as heat conductor. The package according to at least one of the preceding sentences, wherein a waveguide is embedded within a flexible glass substrate, which is preferably attached to a recess in the semiconductor substrate and which preferably has a thickness of less than 200 pm and is most preferably hardened.

The package according to at least one of the preceding sentences, wherein the difference of the CTEs of the glass substrate and the semiconductor substrate is in the range of 0 ppm/K to 6 ppm/K at a Temperature of 25°C.

An electronic device comprising the package according to at least one of the preceding sentences.

Figures and Examples

The invention can be further explained by the figures. Although the figures are all schematic views of envisaged embodiments, they also represent examples in the meaning of the invention. Especially, the dimensions in the figures shall demonstrate the principle and must not match with real devices. Elements of an individual figure can be present in other figures. The description of an element of a figure is also applicable to referring elements in other figures.

This Figures show:

Fig. 1a: Optical Chip to Board coupling with prism recess (top view)

Fig. 1b: Enlarged cross section of Fig. 1a

Fig. 2a: Optical Chip to Board coupling with cut-out butt coupling (top view)

Fig. 2b: Enlarged cross section of Fig. 2a

Fig. 3a: Optical Chip to Board coupling example 2; cut-out butt coupling (top view) Fig. 3b: Enlarged cross section of Fig. 3a

Fig. 4: Optical Chip to Board coupling in another variant

Fig. 5: Example with filled TGVs Fig. 6: Compound optoboard with ultrathin flex-glass-layer

Fig. 7a: Chip to Board coupling with grating coupler (top view)

Fig. 7b: Cross section of Fig. 7a

Fig. 8a: Embodiment with optical bridge (cross section)

Fig. 8b: Isometric view corresponding to Fig. 8a

According to Fig. 1a, which shows an optical chip to board device in the top view, also referred to as optoboard, photo detectors, also referred to as optocoupler (3), can be embedded in the glass substrate (1 ) in a highly precise cut-out to directly align with the embedded waveguides (4) in the glass. The glass itself is most advantageously highly transparent for the wavelength of 1550nm or 980nm, which is suitable for photo detectors and emitters as well.

Fig. 1b shows an enlarged cross section of Fig. 1a, namely in the area of the semiconductor device (2). The photo detector and/or optocoupler (3) can be represented by or can be a part of a semiconductor device, also referred to as chip (2). The waveguide (4) is in this embodiment incorporated within the pane of the glass substrate or glass board (1 ). The optocoupler OC (3) is located at another pane, here the pane of the semiconductor device (3), which comprises in this example a chip.

The signal transmitted in the waveguide (4) is redirected to the optocoupler (3) by an optical device, in this case a recessed prism (3). As said before, the prism can comprise an anti-reflex coating, especially on its entrance surface “butt facet” (6), to mitigate signal loss. Furthermore it can have a reflecting surface “routing facet” (7). In this embodiment, the coating might have a reflectivity of R>0.9 for the communication wavelength X and an incidence angle of 45°.

Further cut-outs can be foreseen within the package, advantageously in the glass substrate (1), to improve the thermal management. Cooling entities can be located in those cutouts, such as a solid copper block, copper foam, etc.. In the embodiment according to Fig. 2a, which again shows the top view of an optoboard, the waveguide (4) and the optocoupler (3) are located at the same pane. Although this is an example which illustrates the principle, it represents the general problem that the optocoupler (3) and/or emitter (3) and thereby the semiconductor substrate (2) need to be precisely aligned.

Fig. 2b shows an enlarged cross section of the area of the optocoupler (3). According to the invention, as for example illustrated in Fig. 2b, the glass substrate (1 ) comprises an aligning or alignment surface, here in form of a cutout, which matches to an aligning surface of the semiconductor substrate (2). Thereby the semiconductor substrate (2) with the optocoupler (3) can be easily mounted to the glass substrate (1 ).

Fig. 3a and 3b represent another embodiment with the matching aligning surfaces. Again, a recess in the glass substrate (1 ) is present. However, other possibilities can be easily derived from the disclosed principle and are subject of the present invention. Especially, different to the principle in Fig. 2a and 2b, the semiconductor device (2), the optocoupler (3) being an element of the semiconductor (2), and the waveguide (4) are within the same pane.

Fig. 4 represents a cross sectional view of another the principle of chip to board coupling, wherein the glass substrate (1 ) itself can comprise more than one part. In the shown embodiment, two glass elements are joined, here by means of laser weld lines (10). The alignment surface can be produced by combining more than one glass elements (1). One glass element, the so called waveguide wafer (9), can comprise the waveguide, the other can represent a carrier wafer (8). The joining of the glass substrates (8, 9) with the laser fusing process resulting in creation of the laser weld lines (10) has the advantage that the waveguide (4) is not effected and that the alignment precision of the waveguide (4) to the optocoupler (3) is enabled, especially because the butt surfaces are precisely aligned and/or remain unaffected by the laser process.

Fig. 5 shows a cross sectional view of another embodiment based on the principle shown in Fig. 4. TGVs (20) through the glass element, here the carrier wafer (8), connect the semiconductor substrate (2) with a surface of the carrier wafer. The TGVs (20) are metallized, here fully filled, in order to provide thermal management capabilities, and represent an electrical conductor. Usually, the carrier wafer can comprise an electrical redistribution layer (22) to which the TGVs are connected. For example, the shown optocoupler (3) can be connected via the TGVs (20) and eventually the redistribution layer (22) to other electronic components. The mean diameter of the TGVs (20) is in the range of 0.5 pm to 50 pm, more preferably 1 pm to 20 pm, most preferably between 2 pm and 10 pm. Ideally the TGVs are cylindrical, but they can also have an hour-glass cross section or a cone like cross section. The metal of the TGV can be Copper, Tungsten, Gold or Silver.

The semiconductor (2) can be connected to the TGVs (20) by an electrical connection, in this case solder (21).

According to the embodiment of Fig. 6, a waveguide is incorporated into a flexible glass layer (90). Such can be provided utilizing an ultra-thin glass, which might have a thickness of about 200 micron or below. Most advantageously, this thin glass layer is hardened in order to improve its mechanical strength. Nevertheless, it is flexible and therefore an be introduced into a recess of the semiconductor substrate (2), which in this case is also provided with the optocoupler (3). The recess so to say serves as plug for the flexible glass substrate. This solution enables a passive alignment of the waveguide and the optocoupler (3), which is removable and/or provide a good coupling efficiency, especially with low loss. Of course, the carrier wafer (8) can be connected to the other components with laser weld lines (10) as shown in the previous figures, or by any other suitable means. Fig. 7a and 7b represent an alternative to the embodiment of Fig. 4. Here, the optical signal being transported within the waveguide (4) is redirected by means of an optical grating (30), which can be applied to the exit or entrance surface of the glass substrate. The optocoupler (3), being mounted to the semiconductor substrate

(2), is aligned to the optical grating (30). The optical grating (30) can in particular be produced by a diffractive structure (Fresnel) etching or laser writing with a short wavelength laser or gray tone lithographic etching.

The embodiment according to Fig. 8a and 8b represent the optical coupling via an optical glass bridge (40) on top of the glass-panel based package. Fig. 8a shows the cross sectional view along the line A-B shown in the isometric view being shown in Fig. 8b. The glass bridge (40) comprises the waveguide (4) and can especially be formed as additive structure to the glass substrate (1 ).

In this embodiment, the semiconductor substrates (2) comprising the optocoupler

(3) are mounted on the surface of the glass substrate (1), which can comprise an electrical redistribution layer (22) to which the optocoupler can be connected.

Further embodiments can be easily derived applying the teaching of this invention.

The invention therefore contributes to replacement of a significant part of the electronic data transport by an optical data transport and by coupling the optical data via fast photo detectors and modulated laser diodes to the electronic data and back and to realize it as close as possible to the semiconductor chiplets.

Legend to the Figures

1 glass board

2 chip

3 optocoupler, OC

4 waveguide

5 recessed prism

6 butt facet

7 routing facet

8 carrier wafer

9 waveguide wafer

10 weld line

20 through glass via (TGV)

21 solder

22 structured conduction layer (e.g. ITO, copper)

30 diffractive and/or holographic coupler

40 optical bridge

90 flex-glass-layer with integrated waveguide

Claims

Patent Claims

1. A package comprising a glass substrate (1 ) and a semiconductor substrate (2), wherein the semiconductor substrate (2) is at least in areas aligned to the glass substrate (1 ).

2. The package according to claim 1 , wherein the glass substrate (1) comprises a waveguide (4), which is coupled to an electronic or optoelectronic device (3) or electric conductor within or attached to the semiconductor substrate (2).

3. The package according to at least one of the preceding claims, wherein the glass substrate (1 ) has an aligning surface, which is matched to an aligning surface of the semiconductor substrate (2).

4. The package according to claim 3, wherein the aligning surface of the glass substrate (1 ) comprises a recess in the glass substrate.

5. The package according to at least one of the preceding claims, wherein the waveguide (4) within the glass substrate (1) is located on another pane than the semiconductor substrate (2); preferably an optical signal being transmittable within the waveguide (4) is redirected to the pane of the semiconductor substrate (1 ) by means of an optical redirection device (5), most preferably a prism, which is especially preferably located in a recess of the glass substrate (1 ).

6. The package according to at least one of the preceding claims, wherein the waveguide (4) within the glass substrate (1) is located on another pane than the semiconductor substrate (2); preferably the optical signal being transmittable within the waveguide (4) is redirected to the pane of the semiconductor substrate (2) by means of an optical grating (30), most preferably a diffraction grating.

7. The package according to at least one of the preceding claims, wherein the glass substrate (1) contains TGVs (20), which preferably are filled with a metal to in order to at least partially serve as heat conductor.

8. The package according to at least one of the preceding claims, wherein a waveguide (4) is embedded within a flexible glass substrate (90), which is preferably attached to a recess in the semiconductor substrate (2) and which preferably has a thickness of less than 200 micron and is most preferably hardened.

9. The package according to at least one of the preceding claims, wherein the difference of the CTEs of the glass substrate (1 ) and the semiconductor substrate (2) is in the range of 0 ppm/K to 2 ppm/K.

10. An electronic device comprising the package according to at least one of the preceding claims.