Bonding fixture

A bonding fixture. In some embodiments, the fixture includes: a plate for supporting a central region of the wafer, the central region including 80% of the area of the wafer; and a frame for supporting: the edge of the wafer, and the edge of the plate, the frame having: a first vacuum passage, for pulling the wafer against an upper surface of the frame, and a second vacuum passage, for pulling the plate against the frame.

FIELD

One or more aspects of embodiments according to the present disclosure relate to fabrication of photonic integrated circuits, and more particularly to a fixture for use in bonding components to a photonic integrated circuit.

BACKGROUND

In a chip-assembly process in which III-V components are bonded to each of a plurality of photonic integrated circuits (PICs) on a wafer, bowing of the wafer may lead to misalignment, e.g., tilt, which may compromise the performance of the PICs.

It is with respect to this general technical environment that aspects of the present disclosure are related.

SUMMARY

According to an embodiment of the present disclosure, there is provided a fixture for holding a wafer, the fixture including: a plate for supporting a central region of the wafer, the central region including 80% of the area of the wafer; and a frame for supporting: the edge of the wafer, and the edge of the plate, the frame having: a first vacuum passage, for pulling the wafer against an upper surface of the frame, and a second vacuum passage, for pulling the plate against the frame.

In some embodiments, the fixture further includes a third vacuum passage, for pulling the wafer against the plate.

In some embodiments, the third vacuum passage is in fluid communication with a groove in the plate.

In some embodiments, the groove in the plate is a straight, diametrical groove in the surface of the plate facing the wafer.

In some embodiments, the plate fits into a recess in the frame, a lower surface of the plate abutting against a shelf at the bottom of the recess.

In some embodiments, the third vacuum passage is connected to a hole in a wall of the recess.

In some embodiments, the upper surface of the plate is below the upper surface of the frame.

In some embodiments, the upper surface of the plate is below the upper surface of the frame by at most 30 microns.

In some embodiments, the second vacuum passage is for pulling the plate against the shelf, and the second vacuum passage is connected to a hole in the shelf.

In some embodiments, the shelf is flat to within 5 microns.

In some embodiments, the upper surface of the frame is flat to within 5 microns.

In some embodiments, the first vacuum passage is connected to a hole in the upper surface of the frame.

In some embodiments, the frame is composed of metal.

In some embodiments, the frame is composed of stainless steel.

In some embodiments, the plate is transparent at a wavelength between 0.8 micron and 11 microns.

In some embodiments, the plate is composed of borosilicate glass.

In some embodiments, the plate has a thickness of between 2 mm and 12 mm.

DETAILED DESCRIPTION

In the process of manufacturing a photonic integrated circuit (PIC) that includes one or more bonded components (e.g., a silicon photonic integrated circuit that includes, bonded to it, III-V components such as III-V lasers or III-V modulators) a laser may be used to bond the components to a wafer including a plurality of such photonic integrated circuits. The photonic integrated circuit may include one or more optical waveguides for guiding light on the photonic integrated circuit, and some of the waveguides on the photonic integrated circuit may be coupled to corresponding waveguides on the bonded components. If the alignment between the waveguides on the photonic integrated circuit and the waveguides on the bonded components is poor, or if excessive stresses are introduced, the performance or reliability of the photonic integrated circuit may be degraded.

If, when the bonding operation is performed, the wafer is not flat, e.g., if it has a bow, tilt and misalignment (and performance degradation of the photonic integrated circuit) may result. A silicon wafer may have a bow of about 60 microns, which may be caused in part by thermal stresses introduced by a soldering process, as discussed in further detail below, and which may be greatest (e.g., have the smallest radius of curvature), near the center of the wafer. This bow, if not compensated for, may cause significant performance or reliability degradation of the photonic integrated circuit. In some embodiments a metal support with ribs may be used to flatten the wafer100(by pressing the ribs against the wafer during the bonding operation). Such a procedure may, however, require that the wafer100be positioned such that the ribs do not obstruct the access of the laser beam to the bonding sites; this positioning operation may be time-consuming. The wafer may have a diameter of between 100 mm and 250 mm.

In some embodiments, therefore, a bonding fixture is used to flatten the wafer100while the bonding operation is performed.FIG.1shows an exploded perspective view of such a fixture, in some embodiments, and of a wafer100that may be used with the fixture. The fixture includes a frame105, a plate110(e.g., a glass window with grooves115, as illustrated and as discussed in further detail below), and a sealing ring120. A plurality of first holes125in an upper surface130of the frame may be employed to pull the wafer100against the upper surface130of the frame105, and a plurality of second holes135in a shelf140at the bottom of a recess145(FIG.2A) in the frame105may be employed to pull the plate110against the shelf140. A plurality of third holes150may be employed to evacuate the space between the plate110and the wafer100.

FIG.2Ais a schematic cross-sectional view of the frame105. The frame105has a central (cylindrical) hole forming an aperture205. The cylindrical recess145, which has a larger diameter than the aperture205, extends to a depth D from the upper surface130of the frame105. The bottom surface of the recess145forms a shelf140, as mentioned above. Each first hole125opens into (e.g., is connected to) a first vacuum passage210(or into a respective first vacuum passage210of a plurality of first vacuum passages210), and each second hole135opens into (e.g., is connected to) a second vacuum passage215(or into a respective second vacuum passage215of a plurality of second vacuum passages215).FIG.2Aalso shows a third hole150in the wall225of the recess145, of a plurality of third holes that may be spaced around the wall225of the recess145, each of which opens into (e.g., is connected to) a third vacuum passage230(or into a respective third vacuum passage230of a plurality of third vacuum passages230). The frame may be composed of metal, e.g., of stainless steel. Each of the holes125,135,150may have a diameter between 1 mm and 2 mm, e.g., about 1.5 mm)

FIG.2Bis a cross-sectional view of the bonding fixture in operation. The plate110fits into the recess145, abutting against the shelf140, and the wafer100abuts against the upper surface130of the frame105. The plate covers each of the second holes135, so that when vacuum is pulled on the second vacuum passage (or passages)215, the plate110is pulled against the frame105(e.g., against the shelf140of the frame105). The wafer100abuts against the upper surface130of the frame105and covers each of the first holes125so that when vacuum is pulled on the first vacuum passage (or passages)210, the wafer100is pulled against the frame105(e.g., against the upper surface130of the frame105). The thickness of the plate110is no greater than the depth D of the recess145, and it may be as much as 20 microns thinner (e.g., 10 microns thinner) than the depth D of the recess145, so that the upper surface of the plate110is at the same height as, or below, the upper surface130of the frame105. As such, the gap between the plate110and the wafer100may, when vacuum is applied and the wafer100is pulled against the plate110, have a width (measured in the vertical direction inFIG.2B) of between 0 microns and 30 microns (e.g., between 0 microns and 20 microns), as shown inFIG.2B. Each of the third holes150opens into a gap235between the wall225of the recess145and the edge of the plate110, so that when vacuum is pulled on the third vacuum passage (or passages)230, the gap235is evacuated, and the grooves115(which are in fluid communication with the third vacuum passage (or passages)230) are evacuated, pulling the wafer100against the plate110. A vacuum having a residual pressure between 50 and 300 mbar may be used, so that the pressure available for flattening the wafer100, if the other surface of the wafer100is exposed to gas (e.g., air) at 1 atmosphere (1000 mbar), may be between 700 mbar and 950 mbar.

The shelf140may be flat to a flatness value between 1 micron and 10 microns (e.g., to 2.5 microns or to 5 microns), so that the plate110, when pulled against the shelf140may be similarly flat. The upper surface130of the frame105may also be flat to a flatness value between 1 micron and 10 microns (e.g., to 2.5 microns or to 5 microns) so that, when the wafer100is pulled against (i) the upper surface130of the frame105and (ii) the plate110, it may be similarly flat. In some embodiments, a circular groove in the shelf connects the second holes135, or a circular groove in the upper surface130of the frame105connects the first holes125. In operation, a laser may illuminate the photonic integrated circuit from below. The substrate of the photonic integrated circuit may be substantially transparent to the wavelength of the laser, which may heat and melt high-temperature solder between the photonic integrated circuit and the bonded components, soldering the bonded components to the photonic integrated circuit. The overlap between the wafer100and the frame105may be sufficiently small that most of the wafer (e.g., between 80% of the area of the wafer100and 99% of the area of the wafer) remains accessible to the laser.

FIGS.3A and3Bshow bottom views of the bonding fixture with (FIG.3A) and without (FIG.3B) the sealing ring120. In the embodiment ofFIGS.3A and3Bthe configuration of the vacuum passages is different from that ofFIGS.2A and2B. As may be seen inFIG.3B, each vacuum passage is a straight hole extending through the frame (and opening, at the top, into a hole125,135,150, visible in the perspective view ofFIG.1). The bottom of the frame105has machined into it a cylindrical recess305into which the sealing ring120may be brazed. The sealing ring120may also be brazed to a cylindrical wall310, so that the volume between the sealing ring120and recessed surfaces315,320forms a vacuum manifold for evacuating the vacuum passages210,215,230. The outer portion of the channel forming the manifold (the portion over the outer recessed surface315) may be deeper than the inner portion of the channel forming the manifold (the portion over the inner recessed surface320); this may make it possible for the first vacuum passage210and the third vacuum passage230to be shorter than they would be if the depth of both portions were the same.

FIGS.4A and4Bshow a plan view and a perspective view, respectively, of the plate110. The plate may be composed of a material that is transparent to the light generated by the laser used for the bonding operation. For example, the plate110may be composed of borosilicate glass. In some embodiments, the laser emits light at a wavelength between 0.8 micron and 11 microns, and the plate110is transparent (e.g., attenuates the light by at most 3 dB on transmission) at the operating wavelength of the laser. The thickness of the plate may be between 2 mm and 12 mm (e.g., it may be about 5 mm). The grooves115may be straight grooves each extending diametrically across the surface of the plate110as shown, or they may be otherwise arranged, e.g., a set of parallel grooves may be employed, or several sets of parallel grooves, at oblique angles to each other, or two sets of parallel grooves at right angles to each other, may be used. The grooves115may have flat, horizontal bottoms, (e.g., each groove115may have a rectangular cross section) and the laser light may be able to propagate through any point on the plate110while preserving sufficiently good beam quality to perform the soldering operation (including in a situation in which a portion of the laser beam is transmitted through a groove115and the remainder of the beam is transmitted through a portion of the plate adjacent to a groove115). The frame105and the plate110may be fabricated to be adequately flat using processes such as grinding, lapping and polishing. The bottom surface of each groove115may be made smooth (so as to transmit the laser beam without unacceptable attenuation or distortion) by polishing using a mechanical polishing method or using etching. The dimensions of the grooves115may be in a range of 0.3 mm to 2 mm wide by 0.3 mm to 3 mm deep. The flatness of the top surface of the plate110may be 2.5 um or better. The plate110may be in different shapes: circular, square, rectangular or any other shape. The number of grooves may be in a range of 1 to 20; e.g., there may be four grooves as illustrated, e.g., inFIGS.4A and4B. The grooves115may be smooth and polished and provide access to the laser beam from the bottom side and directly to the soldering spots.

As used herein, “a portion of” something means “at least some of” the thing, and as such may mean less than all of, or all of, the thing. As such, “a portion of” a thing includes the entire thing as a special case, i.e., the entire thing is an example of a portion of the thing. As used herein, when a second quantity is “within Y” of a first quantity X, it means that the second quantity is at least X−Y and the second quantity is at most X+Y. As used herein, when a second number is “within Y %” of a first number, it means that the second number is at least (1−Y/100) times the first number and the second number is at most (1+Y/100) times the first number. As used herein, the word “or” is inclusive, so that, for example, “A or B” means any one of (i) A, (ii) B, and (iii) A and B.

As used herein, the term “major component” refers to a component that is present in a composition, polymer, or product in an amount greater than an amount of any other single component in the composition or product. In contrast, the term “primary component” refers to a component that makes up at least 50% by weight or more of the composition, polymer, or product. As used herein, the term “major portion”, when applied to a plurality of items, means at least half of the items. As used herein, any structure or layer that is described as being “made of” or “composed of” a substance should be understood (i) in some embodiments, to contain that substance as the primary component or (ii) in some embodiments, to contain that substance as the major component.

Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of “1.0 to 10.0” or “between 1.0 and 10.0” is intended to include all subranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 10.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Similarly, a range described as “within 35% of 10” is intended to include all subranges between (and including) the recited minimum value of 6.5 (i.e., (1−35/100) times 10) and the recited maximum value of 13.5 (i.e., (1+35/100) times 10), that is, having a minimum value equal to or greater than 6.5 and a maximum value equal to or less than 13.5, such as, for example, 7.4 to 10.6. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein.

Although exemplary embodiments of a bonding fixture have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that a bonding fixture constructed according to principles of this disclosure may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof.