GB2340957A - Making proton-exchange 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 a surface of the wafer and (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 paramete e.g. depth of proton exchange 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 e.g. depth of annealing 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, and the analysis steps include the use of prism coupling.

Description

2340957 PROCESS FOR THE MANUFACTURE OF PROTON-EXCHANGED WAVEGUIDES

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to processes for making multi-function integrated optics chips ("MIOCSff) having optimally high electro-optic coefficients, and proton exchanged waveguides, preferably with a rhombohedral structure.

2. Description of the Prior Art

Typical methods for making multifunction integrated optics chips, particularly such chips made of lithium niobate (LiNb03), have included the step of subjecting the chips to proton exchange with acids such as pure benzoic acid at temperatures in the range of about 1500C to about 2500C for times of about 10 to about 50 minutes or more. Where the proton exchange step is carried out at high temperatures or for long times, or both, the lithium niobate substrates tend to form substantial internal stresses that result in microcracking and/or waveguide cross-hatching in the substrate areas exposed to the proton exchange step.

As a result, the chips made by processes including this step frequently exhibit refractive index instabilities, high propagation losses, and lower than desired electro-optic coefficients.

13 6 U.S. Patents Nos. 5,193,2t27- 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 (BL,.,M03), in an altered crystalline phase, known as the 9 phase. This phase (which comprises several sub-phases 91-R4) 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 rl phase.

19 Alternatively, a dilute acid melt with about 2.5% to about 5% 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 (An) of the I final waveguide. Such control becomes increasingly difficult for deeper 2 wayeguides, 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 &n values from wafer to wafer, restrained primarily by the control of process parameters.

6 Furthermore, wafer-to-wafer variabIty 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 hirther 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.

21 SUMMARY OF THE INVENTION

22 The invention provides a method for manufacturing proton 23 exchanged, multifunction integrated optics chips (MIOCs) that are, in 24 preferred embodiments, based on the desired ic or a phases of the rhombohedral qj,.,,NbO3 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.

3 I 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 wavcguides having precisely controllable and reprodudIbIle 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 "tunine', 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 4 I analytical techniques, and then subjecting each wafer to a final stage of 2 the Process step, the duration of which is dictated by the res-ults 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 (An) of the waveguides. The value of An 9 provides information about the crystalline phase attained at that point in 10 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 15 summarized as comprising the following sequence of steps: (a) prm'' 1 9 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 20 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 and the masked wafer analysis step until the first 25 predetermined value has been achieved; (f) removing the mask or mask 26 pattern from the wafer; (g) thermally annealing the wafer, in an oxygen27 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 30 value; and (i) repeating the annealing step and the unmasked wafer 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 ic 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 ("Nomarsid") microscopy. These analytical 14 techniques are well-known in the art, but their utility in the present is 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 An 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, ecd 29 imaging, or measurement from photos. Waveguide surface features, such as crosshatching, can also be imaged and removed by further 6 '-==7 I 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 masldng patern, such as a mask made of a metal such as aluminum, gold, or 6 chromium. Tbese 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 Cuses 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, preferabty 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 250C 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 7 I 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, substantiaUy into and 6 below the surface of the wafer. The heat treatment also converts the 7 phase of the proton exclanged areas of the wafer, where the wafer is 8 lithium niobate, from the initially formad 0 phase to r. or a phase, 9 with low loss and good electro-optical 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 200 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 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.

4 BRIEF DESCRIPTION OF THE DRAWINGS

6 Figure I is a block flow diagram showing a method for making 7 proton-exchangod 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.

13 14 DETAELED DESCRIPTION OF THE INVENTION

Referring now to the drawfngs 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 WiD 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 9 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 (U+) 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 d., 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 stop, 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 An between the proton-exchanged region 32 14 (measured at its surface) and the surrounding wafer 20 (L c., d x An). As will be seen below, the proton-exchanged region 32 will become the 16 waveguideregion in the finished product after completion of the 17 thermal annealing stop.

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 d,,. This time period can be defined as the maximum 24 proton exchange time. In a well-controIled production process, there is usually some variability in the depth and An 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 1 long dilute melt proton exchange processes is a change in the melt 2 composition due to outdiffwion 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-hermeticafly scaled 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 An 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 dp, 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 is 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 At. can be determined from the 19 desired depth by the following formula:

(1) Atp = 0.25(dr12 - dp62)1I:)pl, 21 22 where Atp is the added proton exchange time, d., is the first target 23 proton diffusion depth, dro is the initial proton diffusion depth, and Dp, 24 js 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 d., 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 I waveguides are deed, which require very long proton exchange times 2 in a dilute melt. It has been found from an evaluation of An 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., fl), whereas the original proton exchanged region from the initial pure 6 melt process is most likely a combination of two phases (,S, and.S.).

7 For this reason, two tuning step iterations may be required, since 8 interstitial protons associated with the,82phase move into optically 9 active substitutional sites during the process, adding incrementally to the measured An. 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 prototis are optically active.

14 Anneal Tuning Once the target proton diffusion depth. d., is attained, the 16 mask 22 is removed, and the proton-exchanged wafer 20 passes on to 17, the thermal annealing step 5. There, the unmasked wafer 20 is 18 subjected to thermal annealing at a temperature that gradually increases 19 from about 20C. 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 Mightly before a second (annealing) target proton diffusion depth dAl 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 dAl may be defined at any well-defined point on the 29 concentration curve, In practice, the annealing target proton diffusion depth d.,, is preferably defined at the depth at which the proton.

12 I concentration is e-1 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 d., in the range of about 2 to 5 4 microns is desired, depending on the wavelength for which the 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 EU,.,,Nb03 Optical Waveguides", 5M 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 (ic, or a).

13 At this point, there is usually variability in the proton depth 14 and An due to several factors. One factor can be the variability mi the initial proton exchange depth (the first target depth d,,) and An.

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 nonunifomities in 21 temperature within the oven. In 4 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 An values for the annealed proton exchange regions or waveguide 28 regions 32 by prism coupling. In addition, phase contrast or Nomarsu 29 microscopy is now used to measure the widths of these regions. All of the waveguide regions, or selected re ions, on the wafer 20 can be 91 13 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 d., and An) 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 d,,, 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 At. that can be calculated by an is equation that is similar to Equation (1) above:

16 (2) 4tA 0.25(dAI2 - dAJ)/DA1, 17 where At. is the added thermal anneal time, dAO is the proton diffusion 18 depth after the initial thermal annealing step, ds, is the second 19 (annealing) target depth, and D,, 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 d., with the requisite degree of precision.

23 Equation (2) above can be used to calculate the additional anneal 24. time if the diffusion constants DA, and D. 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 lle or 1/C2depth value. Several sequential tuning processes can be used as precision of the endpoint is required. Additional 14 1 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.

is

Claims (1)

1 T IS CIAIMED IS:

2 1. A method for manufacturing integrated optics chips, 3 comprising the steps of..

4 (a) providing a wafer of a material selected from the group consisting of lithium niobate and lithium tantalate; 6 (b) masking a surface of the wafer to form one or more 7 proton-exchange patterns, of desired size and shape on the 8 masked surface of the wafer, each of the proton exchange 9 patterns comprising an unmasked portion of the masked surface of the wafer; 11 (c) subjecting the unmasked portions of the masked surface 12 to a proton-exchange procedure for a period of time less than a 13 predetermined maximum proton exchange time; 14 (d) analyzing the masked wafer to determine if a specified parameter of the portions subjected to the proton exchange 16 procedure has achieved a first predetermined value; 17 (c) repeating the proton exchange procedure and the 18 masked surface analysis step until the first predetermined value 19 has been achieved; (f) unmasking the wafer; 21 (g) thermally annealing the wafer for a period of time less 22 than a maximum annealing time; 23 (h) anab fAng the unmasked wafer to determine if the 24 specified parameter has achieved a second predetermined value; and 26 (i) repeating the annealing step and the unmasked wafer 27 analysis step until the second predetermined value has been 28 achieved.

29 16 1 1 The method of Claim 1, wherein the specified parameter is the 2 depth of proton diffusion below the surface of the wafer, wherein the 3 first predetermined value of the specified parameter is a first target 4 depth of proton diffusion, and wherein the second predetermined value 5 of the specified parameter is a second target depth of proton diffusion. 6 7 3. The method of Claim 2, wherein the steps of ana4rzing the 8 masked wafer and analyzing the unmasked wafer include the use of 9 prism coupling. 10 11 4. The method of Claim 2, wherein the step of analyzing the 12 unmasked wafer includes the use of phase contrast microscopy. 13 14 S. The method of Claim 3, wherein the step of analyzing the 15 unmasked wafer includes the use of phase contrast microscopy. 16 17 6. Ile method of Claim 1, wherein at least one of the analyzing 18 steps includes the use of infrared spectroscopy. 19 20 7. The method of Claim 1, wherein the proton exchange step 21 includes the step of treating the wafer in a melt comprising benzoic 22 acid. 23 24 8. The method of Claim 2, wherein the step of repeating the proton 25 exchange procedure is performed for a period of time At. determined 26 by the formula 27 Atp = 0.25(dp,2 dpO2)/Dpj, 28 where Atp is the added proton exchange time, dp, is the first target 29 proton diffusion depth, dp, is the initial proton diffusion depth, and D,,, 30 is the expected diffusion constant for the repeated proton exchange 17 procedure.

2 3 9. The method of Claim 2, wherein the step of repeating the 4 annealing step is performed for a period of time '&tA determined by the formula 6 '&tA = 0.25(dA12 - dO2)1I)Al 7 where '&tA 'S the added thermal anneal time, d,, is the proton diffusion 8 depth after the initial thermal annealing step, dA, is the.second target 9 proton diffusion depth, and D,, is the expected diffusion constant for the repeated annealing stop.

11 12 10. The method of Claim 2, wherein concentration of protons in 13 the portions subjected to the proton exchange procedure as a function 14 of depth below the wafer surface after the annealing step approximates a Gaussian distribution from a maximum concentration value.near the 16 surface of the wafer, and wherein the second target depth is measured 17 at the depth at which the proton concentration is approximately lle, 18 times the maximum concentration value.

19 11. The method of Claim 10, wherein the second target depth is in 21 the range of about 2 to about 5 microns.

22 23 12. A method for manufacturing integrated optics chips, 24 oniprising the steps of..

(a) providing a wafer of a material selected from the group 26 consisting of lithium niobate and lithium tantalate; 27 (b) masking a surface of the wafer to form one or more 28 proton-exchange patterns, of desired size and shape on the 29 surface of the wafer, each of the proton exchange patterns comprising an unmasked portion of the surface of the masked 18 1 wafer; 2 (c) subjecting the unmasked portions of the masked wafer to 3 a proton-exchange procedure, by treating the wafer in a melt 4 comprising benzoic acid, for a period of time less than a predetermined maximum proton exchange time; 6 (d) analyzing the masked wafer using prism coupling to 7 determine if the proton diffusion depth in the portions subjected 8 to the proton exchange procedurehas achieved a first target 9 depth value; (e) repeating the proton exchange procedure and the masked 11 wafer analysis step until the first target depth value has been 12 achieved; 13 (f) unmasking the wafer; 14 (g) thermally annealing the wafer for a period of time less than a maximum annealing time; 16 (h) analyzing the unmasked wafer using prism coupling to 17 determine if the proton diffusion depth in the portions subjected 18 to the proton exchange procedure has achieved a second target 19 depth value; and (i) repeating the annealing step and the unmasked wafer 21 analysis step until the second target depth value has been 22 achieved.

23 24 13. The method of Claim 12, wherein the step of analyzing the unmasked wafer includes the use of phase contrast microscopy.

26 27 14. The method of Claim 12, wherein at least one of the analyzing 28 steps includes the use of infrared spectroscopy.

29 15. The method of Claim 12, wherein the step of repeating the 19 1 proton exchange procedure is performed for a period of time At.

2 determined by the formula 3 Atp = 0.25(dp,2 - dpe)ADpI, 4 where Atp is the added proton exchange time, dp, is the first target proton diffusion depth, dpg is the initial proton diffusion depth, and D., 6 is the expected diffusion constant for the repeated proton exchange 7 procedure.

9 16. The method of Claim 12, wherein the step of repeating the annealing step is performed for a period of time At. determined by the 11 formula 12 AtA= 0.25(dA12 - dA,02)/DAII 13 whereAtAis the added thermal anneal time, dAO is the proton diffusion 14 depth after the initial thermal annealing step, dAI is the second target proton diffusion depth, and DA, is the expected diffusion constant for 16 the repeated annealing step.

17 18 17. The method of Claim 12, wherein concentration of protons in 19 the portions subjected to the proton "change procedure as a function of depth below the wafer surface after the annealing step approximates 21 a Gaussian distribution from a maximum concentration value near the 22 surface of the wafer, and wherein the second target depth is measured 23 at the depth at which the proton concentration is approidmately I/e 24 times the maximum concentration value.

26 18. The mahod of Claim 17, wherein the second target depth is in 27 the range of about 2 to about 5 microns.

28 29 19. A method for performing proton exchange in a selected region of a wafer using a melt comprising benzoic acid, the wafer being made I of a material selected from the group consisting of lithium niobate and 2 lithium tantalate, the method comprising the steps of:

3 (a) subjecting the selected region of the wafer to an _initW. proton 4 exchange procedure by treating the wafer in the melt for a period of time less than a predetermined maximum proton "change time; 6 (b) analyzing the selected region to determine if a specified 7 parameter of the selected region has achieved a predetermined value; 8 and 9 (c) repeating the proton exchange procedure and the analyzing step until the predetermined value has been achieved.

11 12 20. The method of Claim 19, wherein the specified parameter is the 13 depth of proton diffusion below the surface of the wafer, and wherein 14 the predetermined value of the specified parameter is a target depth of proton diffusion.

16 17 21. The method of Claim 20, wherein the analyzing step includes 18 the use of prism coupling.

19 22. The method of Claim 20, wherein the step of repeating the 21 proton exchange procedure is performed for a period of time At.

22 determined by the formula 23 Atp 0.25(dp,2 - d?O)/Dpl, 24 where At,, is the added proton exchange time, d., is the first target proton diffusion depth, c6 is the initial proton diffusion depth, and Dp, 26 is the expected diffusion constant for the repeated proton exchange 27 procedure.

28 29 23. A method of thermally annealing a proton-exchanged region of a wafer formed of a material selected from the group consisting of 21 I lithium niobate and lithium tantalate, comprising the steps of:

2 (a) thermally annealing the wafer initially for a period of time 3 less than a maximum annealing time; 4 (b) analyzing the proton-exchanged region to determine if the specified parameter has achieved a predetermined value; and 6 (c) repeating the annealing and analyzing steps until the 7 predetermined value has been achieved.

8 9 24. The method of Claim 23, wherein the specified parameter is the depth of proton diffusion below the surface of the wafer, and wherein 11 the predetermined value of the specified parameter is a target depth of 12 proton diffusion.

13 14 25. The method of Claim 23, wherein the analyzing step includes the use of prism coupHng.

16 17 26. The method of Claim 23, wherein the analyzing step includes 18 the use of phase contrast microscopy.

19 27. The method of Claim 24, wherein the step of repeating the 21 annealing step is performed for a period of time AtAdetermined by the 22 formula 23 4tA= 0.25(dA12 - d4)/DAII 24 where AtAis the added thermal anneal time, dAO is the proton diffusion depth after the initial thermal anneaUng step, dA, is the second target 26 proton diffusion depth, and DA, is the expected diffusion constant for 27 the repeated annealing step.

28 29 28. The method of Claim 24, wherein concentration of protons in the proton-exchanged region as a function of depth below the wafer 22 I surface after the annealing step approximates a Gaussian distribution 2 from a maximum concentration value near the surface of the wafer, and 3 wherein the target depth is measured at the depth at which the proton 4 concentration is approximately I/e times the maximum concentration value.

6 7 29. The method of Claim 24, wherein the target depth is in the 8 range of about 2 to about 5 microns.

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