Wafer-scale thin-film titanium:sapphire photonics

Abstract

Ti-doped sapphire on arbitrary substrates is provided by Ti-doping an undoped sapphire wafer, then layer transferring part of the resulting doped surface region to another substrate. The Ti-doping can be laterally uniform or laterally patterned. Improved control over the vertical doping uniformity and / or lateral doping contrast can be provided by appropriately positioning the top and bottom surfaces of the layer to be transferred relative to the doping profile. For well-characterized doping profiles this can be done by defining the depths of the top and bottom surfaces of the layer to be transferred, since the characterization can relate doping concentration to depth.

Description

[0001] Wafer-Scale Thin-Film Titanium : Sapphire Photonics

[0002] FIELD OF THE INVENTION

[0003] This invention relates to fabrication of Ti-doped sapphire layers on a substrate .

[0004] BACKGROUND

[0005] Integrated Titanium-doped Sapphire ( Ti : Sapphire ) photonics has the potential to change the landscape of integrated visible and near- IR laser sources . Ti : Sapphire is an excellent gain material , with broadband emission, high power-handling, and a large bandgap . However, current approaches towards integrated Ti : Sapphire technology rely on highly doped bulk Ti : Sapphire wafers . Moreover, growth of highly-doped Ti : Sapphire boules that preserve a high figure of merit ( FOM) is costly, and are limited in si ze and doping control as well as uni formity . Although there has been some consideration of layer trans fer processes for Ti-doped sapphire , reports to date all relate to the use of bulk- doped Ti-sapphire . Accordingly, it would be an advance in the art to provide improved fabrication of Ti-doped sapphire disposed on arbitrary substrates . SUMMARY

[0006] In this work, we show that by utili zing deposition, lithography, di f fusion and annealing techniques to convert undoped sapphire substrates into surface-doped substrates , wafer-scale production of integrated Ti : Sapphire-on- Insulator photonics becomes possible .

[0007] In one example , the process begins with a standard, undoped, sapphire substrate . A near surface level source of Titanium is created via implantation of Titanium ions , or via deposition of Titanium metal or Titanium Oxide ( TiOx) . This surface area can be patterned via masked implantation, or lithographically deposited Ti or TiOx. High temperature annealing in a reducing or inert atmosphere is performed to improve the quality of the doping layer, allowing for formation and di f fusion of Ti3+substitutional ions in the sapphire crystal lattice . The damaged layer, as a result of implantation- or di f fusion-related surface defects , is subsequently removed, either through a mechanical process like grinding, a chemical process like reactive plasma etching, or a combination through chemo-mechanical polishing, leaving a pristine surface layer of Ti : Sapphire at the top of the sapphire substrate . The surface-doped Ti : Sapphire is then bonded to a substrate , where the doped region lies at the interface . Then, via a thinning process ( grinding, CMP, etching) , the undoped sapphire portions of the original material are removed, leaving a wafer-scale thin- film Ti : Sapphire platform .

[0008] With wafer-scale Ti : Sapphire thin- films , high- performance lasers and ampli fiers on chip can be made at scale . These lasers can be used in a variety of markets and applications , such as quantum computing, PNT (position, navigation and timing) , and biomedical applications . Ampli fiers can also be used both with integrated Ti : Sapphire lasers for higher output powers, or with other laser technologies, for data communications or high-power ultrafast pulsed lasers.

[0009] This approach could allow for full-scale production of integrated Ti: Sapphire photonics, at a low cost. The cost of a 100 mm sapphire wafer is $60-80. Ti:Sapphire wafers of this size are not produced at all, and the only vendor known to us who was willing to produce a custom wafer quoted $33,000. Thus, costs can be brought down, compared to a bulk Ti: Sapphire wafer, by orders of magnitude. This is so because the thin-film approach necessitates that the majority of the sapphire wafer is removed, requiring only doping the portion of the wafer that will be left in the end. Additionally, doping control becomes lithographically precise, can be custom-tailored, and can even be patterned lithographically, for simultaneous Ti: Sapphire and undoped sapphire photonics. This is important because for some applications in ultra-low-loss photonics, even state of the art FOM for Ti: Sapphire begins to limit the circuit performance .

[0010] BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIGs. 1A-F shows a first embodiment of the invention.

[0012] FIGs. 2A-G show a second embodiment of the invention.

[0013] FIGs. 3A-B and 4A-B show use of a capping layer for annealing .

[0014] FIG. 5 shows an option for the substrate.

[0015] FIGs. 6A-B show a first example of a relation between target layer position and doping profile.

[0016] FIGs. 7A-E show a second example of a relation between target layer position and doping profile. FIGs. 8A-B show some dark-field microscopy characterization images.

[0017] DETAILED DESCRIPTION

[0018] FIGs. 1A-F shows a first embodiment of the invention, which is a method of making a Ti-doped structure on a substrate. In this example, FIGs. 1A-B show Ti-doping an undoped sapphire wafer 102 using surface processing to create a doped surface region 106. More specifically, 104 on FIG. 1A is titanium-containing compounds on the top surface of sapphire wafer 102 or implanted Ti ions. The result of annealing is shown on FIG. IB where 106 is the doped surface region. Here "surface processing" is any processing method that dopes the top surface of the sapphire wafer, regardless of any incidental (and irrelevant) doping that may occur on other parts of the sapphire wafer.

[0019] FIG. 1C shows the result of removing a top portion of the doped surface region 106 to expose a first surface 108' of the doped surface region 108. The doped surface region is referenced as 108 here because it is thinner than it was on FIG. IB. How much material is removed in this step depends on details of the process. It is always important to remove material that is roughened or otherwise damaged by the surface processing for Ti-doping. In some cases, it may be desirable to remove more material than this, and in other cases just this minimal amount of material (typically on the order of hundreds of nanometers to several microns) is removed .

[0020] FIGs. 1D-E show the result of bonding the first surface 108' to a substrate 112 to provide a bonded structure (FIG. IE) having the doped surface region 108 sandwiched between the undoped sapphire wafer 102 and the substrate 112 .

[0021] FIG . I F shows the result of removing the undoped sapphire wafer 102 from the bonded structure to expose a second surface 110 ' of the doped surface region, thereby providing a Ti-doped layer 110 disposed on substrate 112 . The Ti-doped layer is referenced as 110 here because it is thinner than doped surface region 108 as on FIG . IE .

[0022] In this example , the resulting Ti-doped layer 110 has laterally uni form Ti-doping, which results from laterally uni form Ti-doping in the doped surface region 106 .

[0023] FIGs . 2A-G show a second embodiment of the invention . This example is similar to the example of FIGs . 1A-F except that the Ti-doping is laterally patterned . More speci fically, 202 on FIG . 2A is a lateral pattern of titanium-containing compounds on the top surface of sapphire wafer 102 or a lateral pattern of implanted Ti ions . The result of annealing is shown on FIG . 2B where 204 is the resulting laterally patterned doped surface region .

[0024] FIG . 2C shows the result of removing a top portion of the doped surface region 204 to expose a first surface 206 ' of the doped surface region 206 . The doped surface region is referenced as 206 here because it is thinner than it was on FIG . 2B .

[0025] FIGs . 2D-E show the result of bonding the first surface 206 ' to a substrate 112 to provide a bonded structure ( FIG . 2E ) having the doped surface region 206 sandwiched between the undoped sapphire wafer 102 and the substrate 112 .

[0026] FIG . 2 F shows the result of removing the undoped sapphire wafer 102 from the bonded structure to expose a second surface 208 ' of the doped surface region, thereby providing a Ti-doped layer 208 disposed on substrate 112. The Ti-doped layer is referenced as 208 here because it is thinner than doped surface region 206 as on FIG. 2E.

[0027] In this example, the resulting Ti-doped layer 208 has laterally patterned Ti-doping, which results from laterally patterned Ti-doping in the doped surface region 204. FIG. 2G shows a top view of an example of laterally patterned Ti-doping, where the dark squares are the Ti-doped parts of the pattern.

[0028] There are several options for the Ti-doping. In preferred embodiments the Ti-doping is done by providing Ti to the undoped sapphire wafer and then annealing. Suitable methods for providing Ti to the undoped sapphire wafer include, but are not limited to: surface depositing Ti, surface depositing Ti oxide, and implanting Ti ions. The annealing conditions used in our tests are 1600C in argon atmosphere for 1 hour (limited by our furnace) , but we expect a higher temperature (1750C) for a similar or slightly longer time (1.5 hours) may be more optimal. In general, the annealing is preferably done at temperatures above 1100 C, and more preferably above 1300 C.

[0029] FIGs. 3A-B and 4A-B show use of a capping layer for annealing. Optionally, a capping layer can be used for the annealing. Such a capping layer is deposited after the Ti is provided to the undoped sapphire wafer and before the annealing is done. FIGs. 3A-B show an example of a capping layer 302 used for annealing with laterally uniform doping. FIGs. 4A-B show an example of a capping layer 302 used for annealing with laterally patterned doping. Suitable materials for the capping layer include, but are not limited to: Ti oxide, Al oxide, and Ti nitride. In one example, the addition of an AI2O3 capping layer on top of Ti diffusion layer reduces the migration of the surface Ti+Sapphire composite. It may also reduce the lateral surface-based diffusion that washes out the patterned doping. It has been confirmed through verification with wafers cut at 90 degrees to the standard crystal axis (c-axis) , that lateral diffusion is not caused by anisotropic bulk propagation of Ti but via surface channels.

[0030] In both cases, however, subsurface damage is inferred from dark field image characterization, and its removal requires both polishing (chemomechanical polishing) for replanarization as well as plasma etching by hundreds of nanometers, up to several micrometers to ensure that the thin film of Ti-doped sapphire that is produced will be of high crystalline quality. Here FIGs. 8A-B show the relevant dark field images, with FIG. 8A showing results from annealing of 120 nm of Ti only, and FIG. 8B showing results of annealing 120 nm of Ti covered by a 200 nm AI2O3 capping layer .

[0031] The following diffusion sources have been shown to work similarly to produce highly doped near-surface layers in sapphire :

[0032] 20 nm Ti

[0033] 120 nm Ti (with higher surface imperfections)

[0034] 35 nm TiCt, sputtered

[0035] 120 nm Ti capped with 35 nm TiCt

[0036] 20 nm Ti capped with 200 nm AI2O3

[0037] 120 nm Ti capped with 200 nm AI2O3

[0038] 35 nm TiCt capped with 200 nm AI2O3

[0039] 20 nm Ti capped with 60 nm TiN 35 nm TiCt capped with 60 nm TiN

[0040] Practice of the invention does not depend critically on the composition or structure of the substrate. For example, suitable substrates include, but are not limited to: quartz, fused silica, silicon, sapphire, insulator on silicon, insulator on sapphire, and materials having a refractive index of 1.7 or less. FIG. 5 shows an example of insulator 504 (e.g., SiCt) on silicon 502. Insulator on sapphire would have a similar structure, perhaps with a different insulator. In general, any substrate having a top surface suitable for bonding to sapphire can be employed. Typically this amounts to a surface roughness of 1 nm (root-meansquare) or less. Another substrate option is for the substrate to already include photonic circuit elements, and for these photonic circuit elements to thereby become integrated with Ti-doped sapphire photonic circuit elements as a result of this layer transfer process and subsequent processing .

[0041] An important aspect of this work is the ability, in preferred embodiments, to define a relation between the first and second surfaces and the Ti-doping profile provided by the surface processing. FIGs. 6A-B show a first example of this, where first surface 602 and second surface 604 are located at predetermined positions relative to the doping profile of FIG. 6A. Target layer 606 is the region between surfaces 602 and 604. FIG. 6B schematically shows target layer 606 within the doped surface region 106 (top) and the resulting final configuration of target layer 606 disposed on substrate 112 (bottom) . Note the reversal in the order of surfaces 602 and 604 from the top to the bottom of FIG. 6B, consistent with the above-described layer transfer process . For example , the predetermined positions ( of 602 and 604 ) can be selected such that a thickness of the Ti-doped layer is 10 m or less and such that a Ti concentration in the Ti-doped layer varies by + / -50% or less relative to a Ti concentration at the middle of the Ti-doped layer .

[0042] As another example , the predetermined positions ( of 602 and 604 ) can be selected such that a thickness of the Ti- doped layer is 2 pm or less and such that a Ti concentration in the Ti-doped layer varies by + / -30% or less relative to a Ti concentration at the middle of the Ti-doped layer .

[0043] FIGs . 7A-E show a second example of a relation between target layer position and doping profile . Here the Ti- doping is laterally patterned ( FIG . 7A) , so the target layer shown on FIG . 7B includes a doped part 702 and an undoped part 704 , both of which are within surface doped region 204 . FIG . 7C shows the resulting final configuration of layer 208 disposed on substrate 112 . FIG . 7D shows the relation of surfaces 602 and 604 to the doping profile for the doped part 702 of layer 208 . FIG . 7E shows the relation of surfaces 602 and 604 to the doping profile for the undoped part 704 of layer 208 . Note that the depths of surfaces 602 and 604 are the same on FIGs . 7D-E and it is the doping profiles that are di f ferent on the two figures because of the lateral doping pattern . Another noteworthy point is that FIG . 7B schematically shows surface doping above undoped part 704 , consistent with FIG . 7E . Doping behavior like this can occur because Ti can di f fuse preferentially along the surface of a sapphire wafer as indicated above .

[0044] The predetermined positions ( of 602 and 604 ) can be selected such that a thickness of the Ti-doped part 702 is 10 m or less and such that a Ti concentration in the Ti- doped part 702 vertically varies by + / -50% or less relative to a Ti concentration at a vertical center of the Ti-doped part 702.

[0045] As another example, the predetermined positions (of 602 and 604) can be selected such that a thickness of the Ti- doped part 702 is 2 m or less and such that a Ti concentration in the Ti-doped part 702 vertically varies by + / -30% or less relative to a Ti concentration at a vertical center of the Ti-doped part 702.

[0046] A new feature of the example of FIGs. 7A-E is control over the lateral doping contrast. For example, the predetermined positions (of 602 and 604) can be selected such that a lateral doping contrast of the doped surface region (e.g., the contrast between doped part 702 and undoped part 704) is 3:1 or more.

[0047] Preferably in all these cases where surfaces 602 and 604 are defined relative to the doping profile, the surface doping process is sufficiently well-characterized that the doping profile is well-known as a function of depth. In such cases, one can work backward from any desired relation of surfaces 602 and 604 to the doping profile to the corresponding relation of surfaces 602 and 604 to depth. After that, a layer transfer process designed to provide the proper depths for surfaces 602 and 604 will cause these surfaces to also have the intended relation to the doping profile .

Claims

CLAIMS1 . A method of making a Ti-doped structure on a substrate , the method comprising :Ti-doping an undoped sapphire wafer using surface processing to create a doped surface region; removing a top portion of the doped surface region to expose a first surface of the doped surface region; bonding the first surface to a substrate to provide a bonded structure having the doped surface region sandwiched between the undoped sapphire wafer and the substrate ; and removing the undoped sapphire wafer from the bonded structure to expose a second surface of the doped surface region, thereby providing a Ti-doped layer disposed on the substrate .2 . The method of claim 1 , wherein the Ti-doping is laterally uni form in the doped surface region .3 . The method of claim 2 , wherein the first and second surfaces are located at predetermined positions relative to a Ti-doping profile provided by the surface processing .4 . The method of claim 3 , wherein the predetermined positions are selected such that a thickness of the Ti-doped layer is 10 m or less and such that a Ti concentration in the Ti-doped layer varies by + / -50% or less relative to a Ti concentration at the middle of the Ti-doped layer .5 . The method of claim 4 , wherein the predetermined positions are selected such that a thickness of the Ti-dopedlayer is 2 m or less and such that a Ti concentration in the Ti-doped layer varies by + / -30% or less relative to a Ti concentration at the middle of the Ti-doped layer .6 . The method of claim 1 , wherein the Ti-doping is laterally patterned in the doped surface region .7 . The method of claim 6 , wherein the first and second surfaces are located at predetermined positions relative to a Ti-doping profile provided by the surface processing .8 . The method of claim 7 , wherein the predetermined positions are selected such that a thickness of the Ti-doped layer is 10 pm or less and such that a Ti concentration in the Ti-doped layer vertically varies by + / -50% or less relative to a Ti concentration at a vertical center of the Ti-doped layer .9 . The method of claim 8 , wherein the predetermined positions are selected such that a thickness of the Ti-doped layer is 2 pm or less and such that a Ti concentration in the Ti-doped layer vertically varies by + / -30% or less relative to a Ti concentration at a vertical center of the Ti-doped layer .10 . The method of claim 8 , wherein the predetermined positions are selected such that a lateral doping contrast of the doped surface region is 3 : 1 or more .

11. The method of claim 1, wherein the substrate includes a material selected from the group consisting of: quartz, fused silica, silicon, sapphire, insulator on silicon, insulator on sapphire, and materials having a refractive index of 1.7 or less.

12. The method of claim 1, wherein the Ti-doping comprises: providing Ti to the undoped sapphire wafer by a method selected from the group consisting of: surface depositing Ti, surface depositing Ti oxide, and implanting Ti ions; and annealing .

13. The method of claim 12, wherein the annealing is performed at a temperature of 1100C or higher.

14. The method of claim 12, wherein the Ti-doping further comprises depositing a capping layer after the providing Ti to the undoped sapphire wafer and before the annealing.

15. The method of claim 14, wherein the capping layer includes a material selected from the group consisting of: Ti oxide, Al oxide, and Ti nitride.

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