RECTANGULAR CAVITY OPTICAL BEAM SHAPER AND METHOD OF MANUFACTURING THE SAME
A light beam shaper has a rectangular cavity bounded by wafer sidewalls and top and bottom caps. The top and bottom caps are wafer-bonded onto opposing surfaces of the wafer. A reflective coating covers inner surfaces of the cavity. A geometry of the cavity changes a light beam entering the cavity into a rectangular beam profile with uniform spatial intensity at the output.
The present disclosure is directed to a rectangular cavity optical beam shaper. In one embodiment, a light beam shaper has a rectangular cavity bounded by wafer sidewalls and top and bottom caps. The top and bottom caps are wafer-bonded onto opposing surfaces of the wafer. A reflective coating covers inner surfaces of the cavity. A geometry of the cavity changes a light beam entering the cavity into a rectangular beam profile with uniform spatial intensity at the output.
In another embodiment, an article of manufacture includes a semiconductor wafer with a plurality of elongated channels separated by elongated walls. First and second wafers are bonded to opposing sides of the semiconductor wafer. The first and second wafers enclose the elongated gaps to form rectangular cavities. The semiconductor wafers and first and second wafers being diced along a plurality of parallel lines that are at a non-zero angle to a longitudinal axis of the elongated gaps to form a plurality of optical beam shapers. A reflective coating covers at least inner surfaces of the cavities of the optical beam shapers.
In another embodiment, a method involves polishing opposing surfaces of a crystalline wafer. The opposing surfaces have a first crystal orientation. A metal mask is deposited on one of the opposing surfaces. The metal mask is patterned to define openings. Elongated channels are etched in the crystalline wafer through the openings. Edges of the elongated channels form sidewalls having a second crystal orientation resulting in the sidewalls being at right angles to the opposing surfaces. First and second wafers are bonded to opposing surfaces of the crystalline wafer to form a wafer stack. The first and second wafers enclose the elongated channels. The wafer stack is diced along parallel lines that are at a non-zero angle to a longitudinal axis of the elongated channels to form a plurality of light beam shapers. The enclosed elongated channels form cavities of the light beam shapers. A reflective coating is applied to the inner surfaces of the cavities.
These and other features and aspects of various embodiments may be understood in view of the following detailed discussion and accompanying drawings.
The discussion below makes reference to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures. The drawings are not necessarily to scale.
The present disclosure relates to optical devices that change the shape of an input light beam profile and that distribute the power density uniformly across the output profile. These devices, sometimes referred to as optical beam shapers, receive light that has a non-uniform intensity distribution over an input cross-section of the light path. The output of the optical beam shaper has uniform intensity distribution over an output cross-section. Both input and output cross-sections are defined herein as being normal to the light propagation direction and may have any shape, e.g., circular, elliptical, square, rectangular, depending on the end-use application. In this disclosure, shapers are disclosed that provide a rectangular beam output that may be used for applications such as imaging and printing.
Light 105 exiting the beam shaper 104 has a uniform spatial distribution across the cross-section, as indicated by cross-sectional intensity plot 112. In this example, the rectangular output of the beam shaper 104 is different from the circular cross-section of the fiber 102. The geometry of beam shaper 104 is designed to perform the beam profile transformation with minimal optical insertion loss. The light exiting the beam shaper 104 is processed via optics section 106 for further processing, such as exposing objects or illuminating beam steering mirrors. The optics section 106 shapes and redirects the light to a media 108, e.g., a thermochromic print medium. In other embodiments, the optics section 106 may instead direct the light to a display element such as a screen or to selectively evaporate a thin liquid at a printing blanket.
Generally, for integration into an optical device, it is desirable that the beam shaper 104 be compact and low cost, yet be fabricated to tight tolerances for good optical performance. Parameters such as cavity dimensions, finish, reflectivity, etc., can have an effect on both the shape and uniformity of the light exiting the beam shaper 104, as well as alignment with other components in the apparatus that couple with the beam shaper.
Existing optical beam shapers are typically based on bulk optics, such as diffusers and fly-eye lenses. Such optical elements can be bulky, cumbersome to align, and costly. Disclosed below are micro-fabricated light beam shapers made using planar microfabrication techniques such as photolithography, chemical etching, wafer bonding, and plating. These devices can be batch fabricated in wafer scale at low cost. The devices are small, self-contained functional units, and can be easily incorporated onto the output ends of optical fiber modules.
As noted above, the sidewalls 206, 208 are made of a crystalline semiconductor (e.g., Si, GaAs, InP, Ge, SiC, GaN). As will be described in detail below, this allows ensuring that the cavities 202, 204 are rectangular if formed by chemical etching, with parallel side walls and parallel top/bottom walls, as well as side walls being at right angles to the top and bottom walls. Perpendicular sidewalls can also be formed by anisotropic dry etching, such as with the use of inductively-coupled plasma etching or chemically-assisted ion beam etching. The top and bottom caps 210, 212 may be made from materials such as a semiconductor or a glass, and the top cap 210 may be made from a different material than the bottom cap 212. The reflective coating on the inside of the cavities 202, 204 may be a highly reflective metal such as Ag. Other metals such as Al or Au may be used for a reflective surface, and reflector material selection may depend, among other things, on the wavelength of light for which the beam shaper is designed. The reflector can also be a composite of metal and dielectric, such as Ag and a phase-matched dielectric over-coat, or it can be purely dielectrics, such as a multi-layer distributed Bragg reflector.
The dimensions of the cavities 202, 204 (e.g., width 220, height 218, and length 216) are selected to produce a spatially uniform output beam. These dimensions can be found, for example, using a ray tracing simulations. In one embodiment, the optimum cavity dimensions are 500 μm wide×250 μm high×5 mm long. In another embodiment, the optimum cavity dimensions are 1 mm wide×250 μm high×7.5 mm long. These design dimensions can affect the process parameters needed for making the devices. In
The fabrication process for a beam shaper according to example embodiments takes advantage of the dependence of wet chemical etching of silicon on crystal orientation. As seen in
The first step in a micro-fabrication process is to grind and polish a <110>-oriented silicon wafer down to the designed height of the micro-shaper light guide. For the 250 μm-high channel micro-shapers described earlier, the target thickness of the thinning step will be 250 μm thick. Also, the wafer top and bottom surfaces are chemically and mechanically processed (e.g., chemical/mechanical planarization, or CMP) to achieve a mirror-like polish for successful etch masking and wafer bonding in subsequent processing steps.
As shown in
After patterning the mask 400 and removing the photoresist, the wafer is etched using, e.g., KOH, to form the through-hole opening 402. In a preferred embodiment, a KOH concentration of 45% in water at a temperature of 90° C. is used. For a Si wafer, the etch rate under this condition is about 60 μm/hour. In
In an alternative embodiment, the through-hole opening is formed by dry etching, where the slope of the sidewalls are defined by dry etching parameters such as pressure, plasma density, plasma species, RF power, or ion energy, rather than by the crystal orientation of the wafer. In this alternative embodiment, straight sidewalls can be realized even for non-<110> oriented wafers.
After the partial or through-hole etching process, the mask 400 is removed by chemical etching. The top and bottom surfaces of the silicon wafer 300 are then capped with planar wafers. An example of this is shown in the diagrams of
In one embodiment, the interfaces between the silicon 300 and first and second wafers 700, 800 are formed by direct wafer bonding using an anodic bonding process. In one embodiment, the bonding process involves heating wafer pairs to about 350° C., and applying about 600 V across the interface, as indicated by voltage source 702. The elevated temperature and high voltage induce ions to migrate across the interface between the two materials, causing formation of ionic bonds that bind the two surfaces together, forming stack 802 after both wafers 700, 800 have been bonded. Ideally, the thermal expansion coefficient of the cap wafers are similar to wafer 300. Typical glass has thermal expansion coefficient that is about 3 times as large as that of silicon at room temperature. The difference in thermal expansion coefficient can be accommodated if the bond strength is strong enough to hold the stress, but could lead to delamination or cracking if the stress becomes too large. We only consider cap materials whose thermal expansion coefficient is less than 30 times that of the base wafer. A preferred embodiment utilizes fluoroboric glass 700, 800 wafers for strong and reliable anodic bonds. In another embodiment the top and bottom wafers 700, 800 are polished silicon wafers. Wafer bonding can achieved, for example, by using fusion bonding of oxidized wafer surfaces or thermo-compression bonding of metal layers deposited on the silicon wafers.
As shown in the perspective view of
The plating chemical 1004 in one embodiment includes a two-component solution that is mixed together just before being forced through the cavities 1002 via a peristatic pump 1006. One of the two components is a AgNO3 solution 1008 prepared by mixing 3 ml of 0.1 M AgNO3 with 1 ml of 0.8 M KOH and dropping NH4OH until the formed brown precipitates dissolve. This mixture is poured into a 50 ml syringe and diluted with 50 cc of hot deionized water to fill the syringe. The other component 1010 is 2 ml of 0.5 M dextrose poured into a 50 ml syringe and diluted with 50 cc of DI water to fill the syringe. It will be understood that the solutions 1008, 1010 can be prepared in any quantities by scaling the ingredients proportionally.
The beam shaper is held in a fixture 1012 while the peristaltic pump 1006 forces the plating solution through the narrow channels at a controlled flow rate of 11 ml/min for 2 min. The two chemical components 1008, 1010 are mixed at junction 1014 right before being pushed through the cavities 1002. If the components 1008, 1010 are pre-mixed earlier, excessive pre-reaction could occur causing silver plating on fixtures and tubings. The process is self-terminating and results in the deposition of about 200 nm of plated silver on the cavity sidewalls.
In one embodiment, the coating process employs an initial sensitization step, where the cavity sidewalls are treated with a 1.55 mM solution of SnCl2 for two minutes prior to plating to improve coating uniformity. In
To protect the coated silver reflector from oxidizing, the reflector can be further coated with a polymer such as polystyrene, which is transparent at visible and near infrared wavelengths. The polystyrene coating can be deposited by flowing polystyrene solution in 3% weight toluene through the channels after the silver plating step. The thickness of the polystyrene can be designed to phase match the wavelength of interest to achieve enhanced reflectivity compared to the reflectivity of metal alone. In other embodiments, the cavities of the beam shapers may be completely filled with a transparent material, e.g., to both protect the silver coating and to prevent contaminants from entering the channel.
The optical performance of the micro-optical beam shaper in operation was characterized by laboratory measurement. The characteristics of the input light are typical of outputs from an optical fiber and possess a circular beam profile that tapers off gradually near the perimeter. The micro-shaper transforms the beam into a rectangular-shaped profile, in this case having a 250 μm high by 500 μm wide cavity. The plots 1200 and 1202 in
The first and second wafers are bonded 1305 to opposing surfaces of the crystalline wafer to form a wafer stack. The first and second wafers enclose the elongated channels. The wafer stack is diced 1306 along parallel lines that are at a non-zero angle to a longitudinal axis of the elongated channels to form a plurality of light beam shapers. The enclosed elongated channels form cavities of the light beam shapers. A reflective coating is applied 1307 to the inner surfaces of the cavities.
A micro-optical beam shaper can be integrated with optical carriers (e.g., waveguides, fibers) to form an integrated unit. In
As seen in the top view of
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.
The foregoing description of the example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Any or all features of the disclosed embodiments can be applied individually or in any combination are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather determined by the claims appended hereto.
1. A light beam shaper, comprising,
- a wafer of crystalline semiconductor comprising a plurality of elongated channels separated by a plurality of elongated walls, the wafer being diced along a plurality of parallel lines that are at a non-zero angle to a longitudinal axis of the elongated channels such that the diced elongated walls form a plurality of wafer sidewalls;
- top and bottom caps wafer-bonded onto opposing surfaces of the wafer to form a rectangular cavity bounded by two of the wafer sidewalls; and
- a reflective coating on inner surfaces of the cavity, wherein a geometry of the cavity changes a light beam entering the cavity into a rectangular beam profile.
3. The light beam shaper of claim 1, wherein the opposing surfaces of the wafer have a crystalline orientation of <110> and surfaces of the wafer sidewalls facing the rectangular cavity have a <111> crystal orientation.
4. The light beam shaper of claim 1, wherein at least one of the top and bottom caps has a thermal expansion coefficient that is less than 30 times a thermal expansion coefficient of the wafer.
5. The light beam shaper of claim 1, wherein at least one of the top and bottom caps comprises at least one of glass, semiconductor, polymer, and metal.
6. The light beam shaper of claim 5, wherein the glass comprises a fluoroboric glass.
7. The light beam shaper of claim 1, wherein the reflective coating comprises one or both of a metal and a dielectric.
8. The light beam shaper of claim 7, wherein the reflective coating comprises the metal overcoated by the dielectric, the dielectric being phase-matched to a wavelength of the light beam.
9. The light beam shaper of claim 7, wherein the reflective coating comprises the metal overcoated by polystyrene.
10. The light beam shaper of claim 7, wherein the dielectric completely fills the cavity.
11. The light beam shaper of claim 1, wherein light coupled into an entry end of the cavity exits with a spatial intensity profile that varies by no more than 35% at an exit end of the cavity.
12. The light beam shaper of claim 1, wherein sides of the wafer sidewalls have a thickness that defines a height of the cavity, the thickness being less than 600 μm.
13. An optical assembly comprising:
- the light beam shaper of claim 1;
- optical fiber; and
- a coupler that aligns the optical fiber to a center of the cavity of the light beam shaper and mechanically secures the optical fiber to the light beam shaper.
14. An article of manufacture, comprising:
- a semiconductor wafer comprising a plurality of elongated channels separated by elongated walls;
- first and second wafers bonded to opposing sides of the semiconductor wafer, the first and second wafers enclosing the elongated channels to form rectangular cavities, the semiconductor wafer and first and second wafers being diced along a plurality of parallel lines that are at a non-zero angle to a longitudinal axis of the elongated channels to form a plurality of optical beam shapers; and
- a reflective coating on at least inner surfaces of the cavities of the optical beam shapers.
15. The article of manufacture of claim 14, wherein the semiconductor wafer comprises crystalline Si with the opposing surfaces having a crystalline orientation of <110>.
16. A method, comprising:
- polishing opposing surfaces of a crystalline wafer, the opposing surfaces having a first crystal orientation;
- depositing a mask on one of the opposing surfaces;
- patterning the mask to define openings;
- etching elongated channels in the crystalline wafer through the openings, edges of the elongated channels forming sidewalls having a second crystal orientation resulting in the sidewalls being at right angles to the opposing surfaces;
- removing the mask;
- bonding first and second wafers to opposing surfaces of the crystalline wafer to form a wafer stack, the first and second wafers enclosing the elongated channels;
- dicing the wafer stack along parallel lines that are at a non-zero angle to a longitudinal axis of the elongated channels to form a plurality of light beam shapers, the enclosed elongated channels forming cavities of the light beam shapers; and
- applying a reflective coating to the inner surfaces of the cavities.
17. The method of claim 16, wherein the bonding of the first and second wafers to the opposing surfaces of the crystalline wafer comprises anodic bonding.
18. The method of claim 16, wherein the mask comprises Ni.
19. The method of claim 16, wherein the elongated channels are etched with KOH.
20. The method of claim 16, wherein applying the reflective coating comprises flowing an AgNO3 solution through the cavities of the optical beam shapers.
21. The method of claim 20, where applying the reflective coating further comprises flowing a SnCl2 solution through the cavities before flowing the AgNO3 solution.
22. The method of claim 16, further comprising applying a transparent polymer coating to the reflective coating.
23. The method of claim 16, wherein etching the elongated channels in the crystalline wafer comprises dry etching.
24. The light beam shaper of claim 1, wherein the top and bottom caps are anodically wafer-bonded onto the opposing surfaces of the wafer