CA2289876A1 - Process for the manufacture of proton-exchanged waveguides - Google Patents

Abstract

A process for the manufacture of proton-exchanged waveguides for multifunction integrated optics chips ("MIOC's") includes the steps of (a) providing a wafer of a material such as lithium niobate; (b) affixing a removable mask to at least one surface of the wafer to form one or more proton-exchange patterns of desired size and shape on the surface of the wafer; (c) subjecting the unmasked portions of the masked wafer to a proton-exchange procedure for a period of time less than a predetermined maximum proton exchange time; (d) analyzing the masked wafer to determine if a specified parameter of the unmasked portions has achieved a first predetermined value; (e) repeating the proton exchange step and the masked wafer analysis step until the first predetermined value has been achieved; (f) removing the mask or mask pattern from the wafer; (g) thermally annealing the wafer for a period of time less than a maximum annealing time; (h) analyzing the unmasked wafer to determine if the specified parameter of the wafer has achieved a second predetermined value; and (i) repeating the annealing step and the unmasked wafer analysis step until the second predetermined value has been achieved. In a preferred embodiment, the proton exchange step is performed by treating the wafer in a melt comprising benzoic acid, the specified parameter is the depth of proton diffusion below the surface of the wafer, and the analysis steps include the use of prism coupling.

Description

This application is a Continuation-in-Part of prior co-pending 6 application Serial No. 09/127,492; filed July 31, 1998.

9 Not Applicable 12 1. Field of the Invention 13 This invention relates to processes for making mufti-function 14 integrated optics chips ("MIOCS") having optimally high electro-optic coefficients, and proton-exchanged waveguides, preferably with a 16 rhombohedral structure.
17 2. Description of the Prior Art 18 Typical methods for making multifunction integrated optics chips, 19 particularly such chips made of lithium niobate (LiNb03), have included the step of subjecting the chips to proton exchange with acids such as 21 pure benzoic acid at temperatures in the range of about 150°C to about 22 250°C for times of about 10 to about 50 minutes or more. Where the 23 proton exchange step is carried out at high temperatures or for long 24 times, or both, the lithium niobate substrates tend to form substantial internal stresses that result in microcracking and/or waveguide cross-26 hatching in the substrate areas exposed to the proton exchange step. As 27 a result, the chips made by processes including this step frequently 28 exhibit refractive index instabilities, high propagation losses, and lower 29 than desired electro-optic coefficients.
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1 U.S. Patents Nos. 5,193,719 - Chang et al. and 5,442,719 - Chang 2 et al. disclose the manufacture of waveguides in lithium niobate and 3 related materials using a single proton exchange step followed by a 4 single thermal annealing step. The proton exchange step, when carried out in a melt of concentrated acid (e.g., benzoic acid), or a dilute melt 6 of acid with a small amount (not more than about 2.5%) of the 7 corresponding lithium salt, produces a layer of protonated lithium 8 niobate (HxL,_xNb03), in an altered crystalline phase, known as the (3 9 phase. This phase (which comprises several sub-phases (31 /34) is characterized by high losses, with poor electro-optic properties. The 11 thermal annealing step, which diffuses the protons deeper into the 12 substrate, results in a waveguide with reduced losses and good electro-13 optic properties. At the same time, several crystalline phase transitions 14 occur. The goal of the process is to obtain a waveguide having a depth, width, refractive index gradient, and crystalline phase that provide low 16 losses and good electro-optic properties. These desired operational 17 characteristics are generally achieved in the a phase and, to a lesser 18 extent, in the x, phase.
19 Alternatively, a dilute acid melt with about 2.5% to about S% of the corresponding lithium salt can be used to produce a layer of 21 protonated lithium niobate substantially in the a phase. This type of 22 proton exchange step eliminates the need for thermal annealing.
23 The above-described methods are characterized by a difficulty in 24 simultaneously and repeatably attaining optimum values of the desired parameters in a production environment with a single proton exchange 26 step and a single thermal annealing step. There are several contributing 27 factors to this problem.
28 First, fine temperature control in both the proton exchange and 29 the thermal annealing steps is required to define the proton dose, proton diffusion depth, and surface refractive index change (fin) of the C:WP:PAT:54707CIP.APL 2 GCD 98-09-CIP

1 final waveguide. Such control becomes increasingly difficult for deeper 2 waveguides, such as those needed for longer wavelengths, for which the 3 proton exchange and anneal times can be very long. The result is a 4 range of proton diffusion depths and 0n values from wafer to wafer, restrained primarily by the control of process parameters.
6 Furthermore, wafer-to-wafer variability in the lithium niobate 7 supply, even within a single boule, is known to exist, and can result in 8 varying proton diffusion rates. This further widens the range of 9 waveguide dosage and dimensional parameters in a production process.
In addition, the annealing step actually comprises two concurrent 11 processes: in-diffusion of protons, and several crystalline phase 12 transitions accompanied by a back-diffusion of lithium ions. The anneal 13 times required for a particular depth at a particular proton dosage are 14 dependent on the phase composition of the starting proton exchange layer. If more than one phase is produced in the proton exchange 16 process, and if the relative amounts vary from wafer to wafer, variability 17 in the final waveguide parameters will result. Even a small amount of 18 initial variability can result in an unacceptable range in the final 19 waveguide parameters for a well-controlled process.

22 The invention provides a method for manufacturing proton-23 exchanged, multifunction integrated optics chips (MIOC's) that are, in 24 preferred embodiments, based on the desired x or a phases of the rhombohedral HxLi,_,~Nb03 structure. These chips have substantially 26 diffused protons, are substantially free of microcracking and of internal 27 stresses that can result in microcracking, and yet have optimally high 28 electro-optic coefficients and low losses. The method is particularly 29 well-adapted for making such chips from proton-exchanged lithium niobate.
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1 The manufacturing method of the present invention is an 2 improvement in the method disclosed and claimed in U.S. Patent No.
3 5,193,136 - Chang et al., which is assigned to the present application, 4 and the disclosure of which is incorporated herein by reference. As disclosed in Chang et al. '136, the prior art method includes the steps 6 of: (a) providing a wafer of a material such as lithium niobate; (b) 7 affixing a removable mask or mask pattern to at least one surface of the 8 wafer to form one or more proton-exchanged patterns of desired size 9 and shape on the surface of the wafer; (c) treating the masked wafer with a proton-exchanging acid such as benzoic acid at a temperature 11 and for a time sufficient to cause substantial proton exchange at and 12 below the unmasked surface of the wafer, but for a time insufficient to 13 create any microcracking or internal stresses that lead to microcracking 14 in the wafer; (d) removing the mask or mask pattern from the wafer;
and (e) thermally annealing the wafer, in an oxygen-containing 16 environment, at a temperature and for a time sufficient to diffuse the 17 hydrogen ions at and near the surface of the wafer substantially below 18 its surface, and for a time and at a temperature sufficient to restore and 19 to optimize the electro-optic coefficient of the wafer.
The present invention is based on the discovery that "tuning" the 21 proton exchange and thermal annealing steps can result in the creation 22 of waveguides having precisely controllable and reproducible 23 dimensional and refractive index parameters, from wafer to wafer in a 24 production environment, where the limiting factors are the preciseness of control of the process parameters and the uniformity of wafer 26 material from wafer to wafer. This "tuning", in turn, is based on the 27 concept that compensation for differing proton diffusion rates during 28 the proton exchange and thermal annealing steps can be achieved by 29 intentionally stopping each of these steps before its intended total duration, evaluating the waveguides on each wafer by one or more C:WP:PAT:54707CIP.APL 4 GCD 98-09-CIP

1 analytical techniques, and then subjecting each wafer to a final stage of 2 the process step, the duration of which is dictated by the results of the 3 analytical evaluation. The final stage of each step will usually be of 4 much shorter duration than the total intended duration, allowing for a more precise control of the target dimensional and refractive index 6 values.
7 The analytical techniques define the depth (d), width, and surface 8 refractive index change (0n) of the waveguides. The value of 0n 9 provides information about the crystalline phase attained at that point in the process step. The tuning technique also requires knowledge of the 11 diffusion constant within the final stage of each of the "tuned" process 12 steps. In a production process, the diffusion constant is usually 13 determined empirically.
14 Thus, the manufacturing method of the present invention can be summarized as comprising the following sequence of steps: (a) providing 16 a wafer of a material such as lithium niobate; (b) affixing a removable 17 mask or mask pattern to at least one surface of the wafer to form one 18 or more proton-exchange patterns of desired size and shape on the 19 surface of the wafer; (c) subjecting the unmasked portions of the masked wafer to a proton-exchange procedure for a period of time less 21 than a predetermined maximum proton exchange time; (d) analyzing the 22 masked wafer to determine if a specified parameter of the unmasked 23 portions has achieved a first predetermined value; (e) repeating the 24 proton exchange step end the masked wafer analysis step until the first predetermined value has been achieved; (f) removing the mask or mask 26 pattern from the wafer; (g) thermally annealing the wafer, in an oxygen-27 containing environment, for a period of time less than a maximum 28 annealing time; (h) analyzing the unmasked wafer to determine if the 29 specified parameter of the wafer has achieved a second predetermined value; and (i) repeating the annealing step and the unmasked wafer C:WP:PAT:54707CIP.APL S GCD 98-09-CIP

1 analysis step until the second predetermined value has been achieved.
2 If a dilute melt proton exchange process is employed which 3 results in the direct formation of the x or a phase with low losses and 4 good electro-optic properties, then the last three steps [annealing step (g), unmasked wafer analyzing step (h), and repeating step (i)] may be 6 omitted. This is usually the case where the melt is benzoic acid with 7 lithium benzoate present in a concentration greater than about 2.5 per 8 cent.
9 In a preferred embodiment of the method, the specified parameter is the proton diffusion depth. The analytical techniques 11 employed in the masked wafer analysis preferably includes prism 12 coupling, and the unmasked wafer analysis preferably includes prism 13 coupling and phase contrast ("Nomarski") microscopy. These analytical 14 techniques are well-known in the art, but their utility in the present invention lies in their use to determine the progress in the desired 16 diffusion process for the particular waveguide set being evaluated.
17 Another technique of value, but not required, is infrared spectroscopy, 18 which indicates, by the position of the 0-H stretching band, the 19 crystalline phase attained. Other non-destructive analytical techniques may also be found to be useful.
21 To use the method of the present invention, it is also required 22 that the wafer have at least one test area or waveguide of sufficient 23 dimensions to employ prism coupling. This gives the parameters of 24 depth and ~n for the area, which can be related empirically or theoretically to those parameters for the channel waveguides patterned 26 on the wafer. Phase contrast or Nomarski microscopy is capable of 27 imaging annealed proton exchanged waveguides. With this tool, the 28 waveguide widths can be measured with use of a reticle, verniers, ccd 29 imaging, or measurement from photos. Waveguide surface features, such as crosshatching, can also be imaged and removed by further C:WP:PAT:54707CIP.APL 6 GCD 98-09-CIP

1 annealing if sufficient allowance in the index and waveguide dimensions 2 remains to do so.
3 In preferred embodiments, the wafers to be converted to MIOC's 4 are covered, on one or more of their surfaces, with a mask or masking pattern, such as a mask made of a metal such as aluminum, gold, or 6 chromium. These masks preferably include a pattern of sufficient size 7 and shape to form a desired pattern of electro-optic channels in the 8 wafers.
9 After masking, the proton exchange process is performed on the lithium niobate wafers by treating them with an acid that causes proton 11 exchange between the hydrogen ions in the acid and the mobile cations, 12 e.g., lithium ions, in the wafers. The acid treatment takes place at 13 elevated temperature, preferably in the range of about 100° C. to about 14 300° C., and for a time in the range of a few minutes up to as many as 30 hours or more, depending on the concentration of the acid melt.
16 (Temperatures greater than about 250°C may require an enclosed 17 system.) The temperatures and times of proton exchange are, in general, 18 sufficient to cause substantial proton exchange in the desired areas of 19 the wafers, but sufficiently short to avoid the creation of stresses that lead to microcracking, or to the formation of microcracks, in the wafer 21 regions exposed to proton exchange.
22 The proton exchange step is controlled as a consequence of the 23 masked wafer analysis step so as to achieve a first predetermined value 24 of the proton diffusion depth that is not more than about 5 microns and preferably not more than about 1 micron below the surface of the 26 wafer, depending on the wavelength at which the waveguide is designed 27 to operate. To this end, the maximum proton exchange time is 28 preferably limited to no more than about 30 hours for a 1% dilute melt 29 of lithium benzoate in benzoic acid.
After proton exchange is complete (i.e., when the first C:WP:PAT:54707CIP.APL 7 GCD 98-09-CIP

1 predetermined value of the proton diffusion depth has been achieved), 2 the mask or masking pattern is removed. Thereafter, the wafer is heat-3 treated, i.e., thermally annealed, in an air environment, at sufficient 4 temperatures and for sufficient times, to diffuse the hydrogen ions, formed at and near the surface of the wafer, substantially into and 6 below the surface of the wafer. The heat treatment also converts the 7 phase of the wafer, where the wafer is lithium niobate, from the initially 8 formed ~i phase to x or a phase, with low loss and good electro-optical 9 properties.
Preferably, the annealing step is carried out for a time and at a 11 temperature sufficient to reduce the proton concentration at the surface 12 of the wafer from a value in the range of about 44%-65%, to a value in 13 the range of about 0%-35% and preferably less than about 20%. As a 14 consequence of the unmasked wafer analysis step, the annealing step is controlled so as to achieve a second predetermined value of the proton 16 diffusion depth of about 2 to 5 microns, depending on the wavelength 17 for which the waveguide is designed, where the second predetermined 18 value is defined as the depth at which the proton concentration is e-1 19 times the surface proton concentration. In broader terms, the annealing step is continued until the electro-optic coefficient of the wafer before 21 proton exchange is substantially recovered, and optical propagation 22 losses caused by proton exchange are reduced or substantially 23 eliminated.
24 In preferred embodiments, the annealing is carried out by gradually increasing the temperature of the annealing from room 26 temperature, e.g., about 20° C., up to a desired maximum temperature, 27 preferably in the range of about 300° C. to about 400° C., over a time 28 period of up to about 30 minutes, but preferably as rapidly as possible.
29 The annealing continues at the maximum temperature for a time sufficient to cause the desired diffusion of protons into the wafer and to C:WP:PAT:54707CIP.APL 8 GCD 98-09-CIP

1 restore the electro-optic coefficient of the wafer to optimum value, i.e., 2 to a value at or near the value of the wafer before exposure to proton 3 exchange and annealing.

BRIEF DESCRIPTION OF THE DRAWINGS
6 Figure 1 is a block flow diagram showing a method for making 7 proton-exchanged integrated optics chips in accordance with a preferred 8 embodiment of the present invention;
9 Figures 2A and 2B represent the proton exchange step of the method of the present invention; and 11 Figure 3 is a cross-sectional view of an embodiment of a product 12 manufactured by the method of the present invention.

Referring now to the drawings, Figure 1 shows, in block diagram 16 form, a manufacturing process or method in accordance with a 17 preferred embodiment of the invention. In this process, there is 18 provided a wafer 20 (shown in cross-section in Figures 2A and 2B), 19 preferably of lithium niobate, or alternatively of lithium tantalate.
(Hereinafter, the process involving a lithium niobate wafer will be 21 described; the process involving a lithium tantalate wafer will be 22 substantially the same.) After cleaning in step 1, the wafer 20 passes to 23 a masking step 2. The masking comprises placing an aluminum, gold, 24 or chromium mask or mask pattern 22 (Figures 2A and 2B), dimensionally controlled by photolithographic techniques, on the lithium 26 niobate wafer 20 to form a waveguide pattern, which may typically be in 27 the form of a Y-junction, for example.
28 From the masking step 2, the masked lithium niobate wafer 20 29 passes on to a proton exchange step 3. There, as seen in Figures 2A
and 2B, an acid melt 24, of either pure acid (typically benzoic acid) or a C:WP:PAT:54707CIP.APL 9 GCD 98-09-CIP

1 dilute acid (typically benzoic acid with lithium benzoate) impinges on 2 the entire surface of the masked wafer 20. However, because the mask 3 22 blocks all masked portions of the wafer 20, a proton (H+ hydrogen 4 ion) exchange for lithium ions (Li+) is confined to the unmasked areas, one of which is designated by the numeral 26 in Figures 2A and 2B.
6 The goal of the proton exchange step 3 is to achieve a first target 7 proton diffusion depth dP, below the surface of the wafer 20, preferably 8 of about 1 micron. In the unmasked area 26, protons enter the wafer 9 20 via path 28 and lithium ions leave the wafer 20 via path 30. The proton-exchanged wafer 20, after this step, includes a proton-exchanged 11 region 32, as shown in Figure 3. This region has a proton concentration 12 or dose that is proportional to the product of its depth d and the 13 refractive index difference ~n between the proton-exchanged region 32 14 (measured at its surface) and the surrounding wafer 20 (i.e., dxOn). As will be seen below, the proton-exchanged region 32 will become the 16 waveguide region in the finished product after completion of the 17 thermal annealing step.
18 Proton Exchange Tuning 19 The process step of proton exchange tuning requires a first proton exchange step designed, with knowledge of the diffusion constant 21 of the particular acid composition 24, to end at or slightly before the 22 period of time required to obtain a first or interim target proton 23 diffusion depth dP,. This time period can be defined as the maximum 24 proton exchange time. - In a well-controlled production process, there is usually some variability in the depth and 0n due to several factors. One 26 factor can be the slight variability in surface composition, cleanliness or 27 roughness from wafer-to-wafer, which can arise from wafer polishing, 28 cleaning, or other treatments prior to proton exchange. Another factor 29 can be variations in mask thickness, which can affect diffusion rates in the narrow, unmasked waveguide channels. Another factor active in C:WP:PAT:54707CIP.APL 10 GCD 98-09-CIP

1 long dilute melt proton exchange processes is a change in the melt 2 composition due to outdiffusion of lithium ions from the wafers and 3 fractional distillation (such as benzoic acid from a lithium 4 benzoate/benzoic acid melt) in a non-hermetically sealed reactor.
The next step in proton exchange tuning is a masked wafer 6 analysis step 4 (Figure 1) that involves the measurement of the proton 7 diffusion depth and ~n values for the proton-exchanged region 32 by 8 prism coupling. If the values, which correspond to a step profile at this 9 point, lie within the limits specified by the desired process, then the wafer can be sent forward in the process to the next step. In other 11 words, if, for example, the measured proton diffusion depth d equals the 12 first (proton exchange) target depth dPl plus or minus a predetermined 13 tolerance, the process passes to a thermal annealing step 5. If the 14 proton-exchanged region 32 is underdosed, then the wafer 20 can be re-dipped into the acid melt 24 for an additional proton exchange "tuning"
16 time period determined by the diffusion constant of the melt and the 17 desired additional proton dose. The process is additive, and the 18 additional proton exchange time OtP can be determined from the 19 desired depth by the following formula:
(1) OtP = 0.25(dP,2 - dp0 )~DP1~

22 where OtP is the added proton exchange time, dP, is the first target 23 proton diffusion depth, dPO is the initial proton diffusion depth, and DP, 24 is the expected diffusion constant for the repeated (tuning) proton exchange step. Several sequential tuning steps can be used as required 26 to achieve the first target depth dPl with the requisite degree of 27 precision.
28 This tuning procedure can also be used where the proton exchanged 29 region is initially formed in a pure acid melt, and then tuned in a dilute melt, which may be advantageous, for example, where very deep C:WP:PAT:54707CIP.APL 11 GCD 98~09-CIP

1 waveguides are desired, which require very long proton exchange times 2 in a dilute melt. It has been found from an evaluation of On values 3 with the use of pure benzoic acid melts that the resulting waveguide, 4 after tuning with a dilute melt process, is most likely a single phase (i.e., ~3,), whereas the original proton exchanged region from the initial pure 6 melt process is most likely a combination of two phases (/31 and J32).
7 For this reason, two tuning step iterations may be required, since 8 interstitial protons associated with the f32 phase move into optically 9 active substitutional sites during the process, adding incrementally to the measured Vin. Unless the concentration of initial interstitial protons can 11 be independently determined, the duration of a single tuning step 12 cannot be accurately calculated. The tuning step duration can only be 13 accurately calculated once all protons are optically active.
14 Anneal Tuning Once the target proton diffusion depth dPl is attained, the 16 mask 22 is removed, and the proton-exchanged wafer 20 passes on to 17 the thermal annealing step S. There, the unmasked wafer 20 is 18 subjected to thermal annealing at a temperature that gradually increases 19 from about 20°C. up to a desired maximum that is preferably in the range of about 300° C. to about 400° C. over a period of up to about 30 21 minutes, but preferably as quickly as possible to achieve precision of the 22 actual anneal time and temperature.
23 The process step of anneal tuning requires a first annealing step 24 designed to end at or slightly before a second (annealing) target proton diffusion depth dA, is attained. The proton concentration after thermal 26 annealing follows a Gaussian or near-Gaussian distribution curve from a 27 maximum value at the surface of the wafer. The second target proton 28 diffusion depth dA, may be defined at any well-defined point on the 29 concentration curve. In practice, the annealing target proton diffusion depth dA, is preferably defined at the depth at which the proton C:WP:PAT:54707CIP.APL 12 GCD 98-09-CIP

1 concentration is e-' times the proton concentration at the surface of the 2 wafer, or alternatively at e-2 times the surface concentration.
3 Preferably, an annealing target depth dA, in the range of about 2 to 5 4 microns is desired, depending on the wavelength for which the S waveguide is designed.
6 It has been found that early in an anneal process, the optical 7 parameters do not follow a square root diffusion law, most likely due to 8 transition through the various phases, as described in Korkishko et al., 9 "The Phase Diagram of I~Li,.,rNb03 Optical Waveguides", SPIE, Vol.
2997, pp. 188-200 (1997). Thus anneal tuning becomes effective as the 11 first anneal process endpoint approaches the final desired waveguide 12 phase (xl or a).
13 At this point, there is usually variability in the proton depth 14 and ~n due to several factors. One factor can be the variability in the initial proton exchange depth (the first target depth dP,) and On.
16 Another factor can be substrate compositional nonuniformities from 17 wafer-to-wafer. Still another factor can be nonuniformity in the 18 temperature control from wafer-to-wafer in the anneal process itself, 19 particularly for long anneal times. This may occur due to fluctuations in the oven temperature itself or to positional nonuniformities in 21 temperature within the oven. In a production process, the incremental 22 cost of improving the thermal uniformity of the anneal process to the 23 precision required must be weighed against the cost of the extra anneal 24 tuning step.
The next step in anneal tuning is an unmasked wafer analysis step 6 26 (Figure 1) that requires the measurement of the proton diffusion depth 27 and 0n values for the annealed proton exchange regions or waveguide 28 regions 32 by prism coupling. In addition, phase contrast or Nomarski 29 microscopy is now used to measure the widths of these regions. All of the waveguide regions, or selected regions, on the wafer 20 can be C:WP:PAT:54707CIP.APL Jl3 GCD 98-09-CIP

1 measured. It is also possible at this point to assess surface stresses on 2 the waveguide regions, which are observed as a crosshatching pattern 3 exhibited on some waveguide regions, which may be due to a thin layer 4 of a higher order phase residual on the surface. This pattern usually disappears with further annealing. Waveguide region widths are noted, 6 and the additional anneal time (which will be calculated to attain the 7 second (annealing) target depth dA, and 0n) can be adjusted so as not 8 to exceed the design for the waveguide during the tuning process.
9 If, after the initial annealing period, the measured proton diffusion depth d equals the annealing target depth dA, plus or minus a 11 predetermined tolerance, the process passes to the final manufacturing 12 steps described below. If the measured depth is less than the annealing 13 target depth, then the wafer 20 can be subjected to thermal annealing 14 for an additional "tuning" time period OtA that can be calculated by an equation that is similar to Equation (1) above:
16 (2) ~tA = 0.25(dA,2 - dAOZ)/DA1~
17 where ~tA is the added thermal anneal time, dAO is the proton diffusion 18 depth after the initial thermal annealing step, dA, is the second 19 (annealing) target depth, and DA, is the expected diffusion constant for the repeated (tuning) annealing step. Several intermediate and 21 sequential tuning steps can be used as required to achieve the second 22 (annealing) target depth dAl with the requisite degree of precision.
23 Equation (2) above can be used to calculate the additional anneal 24 time if the diffusion copstants DA, and DAO for the process are known and if a phase change does not occur.
26 Because the anneal process produces waveguides with a Gaussian or 27 near-Gaussian concentration profile in depth, as noted above, the prism 28 coupling data must be fitted to the appropriate profile to obtain the 29 desired 1/e or 1/e2 depth value. Several sequential tuning processes can be used as precision of the endpoint is required. Additional C:WP:PAT:54707CIP.APL 14 GCD 98-09-CIP

i information, such as that obtained from microscopic examination of the 2 waveguides, can be used to determine whether further anneal time is 3 either needed or contraindicated. The incremental increase in 4 waveguide width with anneal time varies from wafer to wafer, but a good indication of the relative rates can be calculated from the values 6 measured after the first anneal period.
7 Referring again to Figure 1, when thermal anneal tuning is finished, 8 the wafer 20 passes on to a photolithographic step 7 where, using 9 photolithographic processes, electrodes may be formed on the wafer, if desired. The wafer 20 then passes on to a dicing, polishing, and 11 packaging step 8 where the wafer 20 is cut into products of desired size 12 and shape before passing on to a finishing step 9.
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Claims (29)

1. A method for manufacturing integrated optics chips, comprising the steps of:
(a) providing a wafer of a material selected from the group consisting of lithium niobate and lithium tantalate;
(b) masking a surface of the wafer to form one or more proton-exchange patterns, of desired size and shape on the masked surface of the wafer, each of the proton exchange patterns comprising an unmasked portion of the masked surface of the wafer;
(c) subjecting the unmasked portions of the masked surface to a proton-exchange procedure for a period of time less than a predetermined maximum proton exchange time;
(d) analyzing the masked wafer to determine if a specified parameter of the portions subjected to the proton exchange procedure has achieved a first predetermined value;
(e) repeating the proton exchange procedure and the masked surface analysis step until the first predetermined value has been achieved;
(f) unmasking the wafer;
(g) thermally annealing the wafer for a period of time less than a maximum annealing time;
(h) analyzing the unmasked wafer to determine if the specified parameter has achieved a second predetermined value;
and (i) repeating the annealing step and the unmasked wafer analysis step until the second predetermined value has been achieved.

2. The method of Claim 1, wherein the specified parameter is the depth of proton diffusion below the surface of the wafer, wherein the first predetermined value of the specified parameter is a first target depth of proton diffusion, and wherein the second predetermined value of the specified parameter is a second target depth of proton diffusion.

3. The method of Claim 2, wherein the steps of analyzing the masked wafer and analyzing the unmasked wafer include the use of prism coupling.

4. The method of Claim 2, wherein the step of analyzing the unmasked wafer includes the use of phase contrast microscopy.

5. The method of Claim 3, wherein the step of analyzing the unmasked wafer includes the use of phase contrast microscopy.

6. The method of Claim 1, wherein at least one of the analyzing steps includes the use of infrared spectroscopy.

7. The method of Claim 1, wherein the proton exchange step includes the step of treating the wafer in a melt comprising benzoic acid.

8. The method of Claim 2, wherein the step of repeating the proton exchange procedure is performed for a period of time .DELTA.t p determined by the formula .DELTA.t P = 0.25(d P1 2 - d P0 2)/D P1, where .DELTA.t P is the added proton exchange time, d P1 is the first target proton diffusion depth, d P0 is the initial proton diffusion depth, and D P1 is the expected diffusion constant for the repeated proton exchange procedure.

9. The method of Claim 2, wherein the step of repeating the annealing step is performed for a period of time .DELTA.t A determined by the formula .DELTA.t A = 0.25(d A1 2 - d A0 2)/D A1, where .DELTA.t A is the added thermal anneal time, d A0 is the proton diffusion depth after the initial thermal annealing step, d A1 is the second target proton diffusion depth, and D A1 is the expected diffusion constant for the repeated annealing step.

10. The method of Claim 2, wherein concentration of protons in the portions subjected to the proton exchange procedure as a function of depth below the wafer surface after the annealing step approximates a Gaussian distribution from a maximum concentration value near the surface of the wafer, and wherein the second target depth is measured at the depth at which the proton concentration is approximately 1/e times the maximum concentration value.

11. The method of Claim 10, wherein the second target depth is in the range of about 2 to about 5 microns.

12. A method for manufacturing integrated optics chips, comprising the steps of:
(a) providing a wafer of a material selected from the group consisting of lithium niobate and lithium tantalate;
(b) masking a surface of the wafer to form one or more proton-exchange patterns, of desired size and shape on the surface of the wafer, each of the proton exchange patterns comprising an unmasked portion of the surface of the masked wafer;
(c) subjecting the unmasked portions of the masked wafer to a proton-exchange procedure, by treating the wafer in a melt comprising benzoic acid, for a period of time less than a predetermined maximum proton exchange time;
(d) analyzing the masked wafer using prism coupling to determine if the proton diffusion depth in the portions subjected to the proton exchange procedure has achieved a first target depth value;
(e) repeating the proton exchange procedure and the masked wafer analysis step until the first target depth value has been achieved;
(f) unmasking the wafer;
(g) thermally annealing the wafer for a period of time less than a maximum annealing time;
(h) analyzing the unmasked wafer using prism coupling to determine if the proton diffusion depth in the portions subjected to the proton exchange procedure has achieved a second target depth value; and (i) repeating the annealing step and the unmasked wafer analysis step until the second target depth value has been achieved.

13. The method of Claim 12, wherein the step of analyzing the unmasked wafer includes the use of phase contrast microscopy.

14. The method of Claim 12, wherein at least one of the analyzing steps includes the use of infrared spectroscopy.

15. The method of Claim 12, wherein the step of repeating the proton exchange procedure is performed for a period of time .DELTA.t P
determined by the formula .DELTA.t P = 0.25(d P1 2 - d P0 2)/D P1, where .DELTA.t P is the added proton exchange time, d P1 is the first target proton diffusion depth, d P0 is the initial proton diffusion depth, and D P1 is the expected diffusion constant for the repeated proton exchange procedure.

16. The method of Claim 12, wherein the step of repeating the annealing step is performed for a period of time .DELTA.t A determined by the formula .DELTA.t A = 0.25(d A1 2 - d A0 2)/D A1, where .DELTA.t A is the added thermal anneal time, d A0 is the proton diffusion depth after the initial thermal annealing step, d A1 is the second target proton diffusion depth, and D A1 is the expected diffusion constant for the repeated annealing step.

17. The method of Claim 12, wherein concentration of protons in the portions subjected to the proton exchange procedure as a function of depth below the wafer surface after the annealing step approximates a Gaussian distribution from a maximum concentration value near the surface of the wafer, and wherein the second target depth is measured at the depth at which the proton concentration is approximately 1/e times the maximum concentration value.

18. The method of Claim 17, wherein the second target depth is in the range of about 2 to about 5 microns.

19. A method for performing proton exchange in a selected region of a wafer using a melt comprising benzoic acid, the wafer being made

20 of a material selected from the group consisting of lithium niobate and lithium tantalite, the method comprising the steps of:
(a) subjecting the selected region of the wafer to an initial proton-exchange procedure by treating the wafer in the melt for a period of time less than a predetermined maximum proton exchange time;
(b) analyzing the selected region to determine if a specified parameter of the selected region has achieved a predetermined value;
and (c) repeating the proton exchange procedure and the analyzing step until the predetermined value has been achieved.
20. The method of Claim 19, wherein the specified parameter is the depth of proton diffusion below the surface of the wafer, and wherein the predetermined value of the specified parameter is a target depth of proton diffusion.

21. The method of Claim 20, wherein the analyzing step includes the use of prism coupling.

22. The method of Claim 20, wherein the step of repeating the proton exchange procedure is performed for a period of time .DELTA.t P
determined by the formula .DELTA.t P = 0.25(d P1 2 - d P0 2)/D P1, where .DELTA.t P is the added proton exchange time, d P1 is the first target proton diffusion depth, d P0 is the initial proton diffusion depth, and D P1 is the expected diffusion constant for the repeated proton exchange procedure.

23. A method of thermally annealing a proton-exchanged region of a wafer formed of a material selected from the group consisting of lithium niobate and lithium tantalate, comprising the steps of:
(a) thermally annealing the wafer initially for a period of time less than a maximum annealing time;
(b) analyzing the proton-exchanged region to determine if the specified parameter has achieved a predetermined value; and (c) repeating the annealing and analyzing steps until the predetermined value has been achieved.

24. The method of Claim 23, wherein the specified parameter is the depth of proton diffusion below the surface of the wafer, and wherein the predetermined value of the specified parameter is a target depth of proton diffusion.

25. The method of Claim 23, wherein the analyzing step includes the use of prism coupling.

26. The method of Claim 23, wherein the analyzing step includes the use of phase contrast microscopy.

27. The method of Claim 24, wherein the step of repeating the annealing step is performed for a period of time .DELTA.t A determined by the formula .DELTA.t A = 0.25(d A~2 - d A~2)/D A~, where .DELTA.t A is the added thermal anneal time, d A~ is the proton diffusion depth after the initial thermal annealing step, d A~ is the second target proton diffusion depth, and D A~ is the expected diffusion constant for the repeated annealing step.

28. The method of Claim 24, wherein concentration of protons in the proton-exchanged region as a function of depth below the wafer surface after the annealing step approximates a Gaussian distribution from a maximum concentration value near the surface of the wafer, and wherein the target depth is measured at the depth at which the proton concentration is approximately 1/e times the maximum concentration value.

29. The method of Claim 24, wherein the target depth is in the range of about 2 to about 5 microns.