We demonstrate fabrication of periodically poled lithium niobate samples by electric field poling, after patterning by interference lithography. Furthermore we investigate the poling process at an overpoling regime which caused the appearance of submicron dot domains very similar to those induced by backswitch but different in nature. We show the possibility for realizing submicron-scaled three-dimensional domain patterns that could be applied to the construction of photonic crystals and in nonlinear optics. We show that high etch-rate applied to such structures allows to obtain pyramidal-like submicron relief structures which in principle could find application for waveguide construction in photonic bandgap devices.
©2003 Optical Society of America
In recent years, periodically poled ferroelectric materials have gained significant importance in nonlinear optical frequency-conversion processes. First realizations of bulk Periodically Poled Lithium Niobate (PPLN) date back to around 1990 when the possibility of fabricating periodically domain inverted structures at the z-face of Lithium Niobate (LN) by a combination of diffusion and heat treatment close to the Curie temperature was demonstrated [1,2]. Since then, bulk and waveguide LN has found increasing applications in integrated opto-electronic and telecom devices .
Many approaches were investigated to achieve the required periodically reversed domain structure in LN crystal samples. Some of them include periodic modification during crystal growth , surface impurity diffusion , electron-beam writing treatment , and most recently, electric field treatment by applying short electric pulses on finger electrodes patterned by photolithography on the z-face of thin z-cut plates . This technique was further developed by Webjörn and Pruneri at Southampton University [8,9] and by Miller and his coworkers at Stanford University  to a reliable fabrication process for a wafer thickness up to 500µm.
Currently, the electric field poling technique is the most extensively employed by using a series of short high-voltage pulses or delivering the entire poling charge in a single pulse . Although a variety of methods have recently been investigated to improve the pattern fidelity of the reversed domains [12–15], the basic technique of domain patterning in PPLN uses a lithographically patterned insulating layer and lithium chloride liquid electrodes contained in a chuck, sandwiching the sample between two O-ring gaskets. Current monitoring in the external circuit allows control of the domain evolution during poling. One of the main difficulties in PPLN fabrication using the electric field poling technique is the lateral domain spreading occurring in LN during the poling process. Upon application of an external electric field exceeding the coercive field of the material, domains first nucleate along the edges of the electrodes, due to the increase in field strength caused by the fringing field, and then spread laterally underneath the adjacent insulating layer [7,9,16]. Domain wall propagation, away from the electrode edges, makes it difficult to control the duty cycle, although the lithographic pattern can be adjusted to take into account the expected domain spreading.
We use here, for the first time to the best of our knowledge, Interference Lithography (IL) insulating patterns for the electric field periodic poling of 500µm thick LN crystal samples. In Section 2 we illustrate the main features of the IL process and the experimental results obtained for PPLN fabrication by using such IL patterns. The interferogram to be printed on the photoresist-coated crystal samples was generated by a Michelson type interferometric setup. Limitations and advantages concerning the use of IL as a technique for patterning the samples to be poled are discussed. In Section 3 we extend our investigation on the possibility of obtaining submicron sized reversed domain structures by using an electric field overpoling technique and IL insulating patterns again. In conventional electric field poling the external voltage pulse is to be stopped before the current flowing in the circuit drops to zero, indicating the poling progression under the photoresist. We investigate the effects of overpoling which causes the appearance of high density and submicron-scaled single dot domains following the geometry of the insulating pattern. Such dot domains resemble, in some cases, those obtained by Batchko and co-workers [14,17] by using the backswitch poling technique. These preliminary results show also the possibility of fabricating three-dimensional (3D) ferroelectric domain patterns for what is the first time, to our knowledge, paving the way for novel photonic crystals configurations. The electric field overpoling process, unlike the conventional one, gives a less critical control of both the applied voltage pulse width and the insulating pattern duty cycle in order to avoid domains’ spreading under the insulating layer.
In Section 4 we investigate the morphology of the above mentioned 3D domain patterns by etching the samples at different high etch-rates. The differential etch-rate for inverted and non-inverted domains allowed to domain-engineer bulk LN with high regular and smooth submicron pyramidal-shaped relief structures. These structures are similar to those obtained by Eason and co-workers at the Optoelectronic Research Centre (Southampton University) [18–20] by using an etching process or laser ablation. These structures could be of wide interest for optoelectronic applications such as waveguide manufacturing or photonic bandgap devices.
2. Interference Lithography for PPLN fabrication
We here report the results obtained for PPLN fabrication by electric field poling of a LN crystal sample patterned by IL. LN z-cut single domain crystals (by Crystal Technology Inc.) in congruent composition are provided as 3-inch diameter, 500µm thick wafers polished on both sides. After solvent cleaning, the z+ surface was spin-coated with a 1.3µm thick positive photoresist layer (Shipley 1813-J2), baked in a conventional oven at 90°C for 30 minutes and then patterned by IL. IL is a quite simple and inexpensive technique for generating periodic structures over large areas.
2.1 The Interference Lithography Process
IL is carried out by combining two coherent wavefronts to form a sinusoidal intensity pattern in space. The interferogram is created within the volume of space defined by the overlapping beams and, because IL does not need any photolithographic mask, the field size only depends on the size of the two beams, whereas in conventional lithography the imaging system generally limits the workable field size. By exposing a layer of photoresist to this pattern, a simple one-dimensional (1D) grating can be obtained. Furthermore, by overlaying multiple exposures or combining more than two beams, it is possible to generate more complex twodimensional (2D) patterns, including square arrays of dots or holes. IL has some unique advantages over conventional optical lithography. In particular, the spatial resolution of IL can easily exceed the resolution limits of today’s optical steppers when comparable wavelengths are used. For example, structures as small as 100nm are readily patterned by IL using a source at 351nm wavelength [21–22] with the additional advantage of a faster process, if compared to Electron Beam Lithography, especially if large areas need to be exposed. Compared to mask optical lithography the IL is not diffraction limited and the depth of focus for IL is effectively infinite on the scale of planar devices. This makes IL well suited for applications where substrate flatness and topography are critical issues. One more attractive feature of IL is its implementation with relatively simple optical components so that effects due to lens aberrations are dramatically reduced compared to the case of conventional lithography.
In our set-up the interference fringes are generated by the Michelson interferometer shown schematically in Fig. 1. The coherent source is a He-Cd laser delivering a power of 65mW at 441.6nm. The interferometer is designed to produce an interferogram which covers a circular region of about 25mm in diameter on the z+ crystal face with a fringe period of 30µm.
2.2 Periodic poling of IL patterned lithium niobate samples
PPLN fabrication is achieved at room temperature by an electric field poling process. A positive voltage pulse slightly exceeding the coercive field of LN (~21kV/mm) is applied on the z+ patterned crystal face by using a liquid electrolyte consisting of LiCl in deionized water. The liquid electrode configuration has two electrolyte containing chambers which squeeze the sample between two O-ring gaskets, as shown schematically in Fig. 2. Figure 3 illustrates the external electrical circuit. A conventional Signal Generator (SG) drives an High Voltage Amplifier (HVA - 2000x), provided by Trek, Inc., with a series current limiting resistor, RS = 50MΩ, in order to get a 12kV positive voltage. A diode rectifier D is connected to the output of the HVA to prevent flowing of backswitch current in the circuit. It is well known  that, in LN, the reversed domains typically grow beyond the width of the electrodes as the result of the remaining fringing fields along the edges of the lithographic grating strips. For example, in PPLN typically processed for infrared applications (periods > 10µm), the inverted domain width will result to be ~3-4µm wider than that of the electrode. To obtain the desired domain size, insulating strips wider than the electrodes must be fabricated.
The strategy for optimal domain patterning, with conventional electric field poling, is to stop the voltage pulse before poling progresses under the photoresist layer. An in situ method which can be used as a stopping criterion consists in watching for a drop in the poling current Ipol and a corresponding rise in the poling voltage Vpol, both effects indicating that the sample has completely poled under the electrodes and the domains are now laterally spreading under the insulating layer [7,16]. In fact, the conductivity of LN at room temperature is low enough that the poling current can be monitored readily by measuring the voltage drop across the resistance Rm = 10kΩ while a conventional High Voltage Probe (HVP) is used to measure the poling voltage Vpol across the sample. Both current and voltage waveforms are visualized on the oscilloscope Osc during the poling process. Another method for controlling the time length of the voltage pulse is the monitoring of the SHG through the crystal during poling.
We poled periodically a LN crystal sample, patterned by the above mentioned fringes at a period of 30µm, by using the conventional electric field poling technique based on the monitoring of the poling current A mixture of hydrofluoric and nitric acid provides a differential etch-rate for the z+ and z- faces of the crystal domains. Figure 4 shows the patterned side view of the obtained PPLN domain structure revealed by wet etching process at room temperature of 60 minutes in a HF:HNO3=1:2 acid mixture. This image was taken in the central region of the pattern and it is representative of the whole PPLN structure obtained inside an area of about 20mm in diameter. It is important to point out that only the peripheral region of the pattern, outside this area, lacks of homogeneity. In fact the quality of the photoresist grating in the peripheral region is affected by two effects: non-uniform intensity distribution of the interfering beams and non-perfect superposition of the two interfering beams. It is worth noting that the PPLN area can extended by increasing the exit pupil aperture of the interferometer.
Figure 5 shows the surface profilometric measurement performed by a conventional KLA alpha-step Surface Profiler on the patterned side of the periodically poled sample after etching. The measurement was taken along the direction of the x main axis of the crystal.
The profilometric measurement provides an estimation of the period of the revealed domains around 30µm, in good agreement with the expectations. The observation of both the optical micrograph in Fig. 4 and the profile measurement in Fig. 5 leads to the conclusion that the duty cycle of the obtained periodic domain structure is not exactly at 50%, as required for maximum efficiency in nonlinear frequency conversion applications. It can be noted the lack of periodic regions and some random variation in the position of the domain walls. This happens because, in the case of IL, we are not able to control the duty cycle of the fringes to be printed on the photoresist-coated sample, as is usually done in conventional lithography by a suitable design of the lithographic mask. We believe that the use of negative photoresist may overcome this inconvenient, taking into account that the bright fringes of the interferogram are wider than the dark ones. Related experiments are in preparation. Anyway, it is evident that the homogeneity and periodicity of the domain pattern obtained here is comparable to other results reported in literature [4,7,9,11,23] and obtained using conventional photolithography.
3. Appearance of submicron dot domains by an overpoling process
In this section we report the results obtained using the electric field poling technique in the overpoling regime, for crystal samples patterned by 1D and 2D IL gratings.
3.1 Electric field overpoling of LN sample using one-dimensional IL pattern
Electric field overpoling was applied to a LN crystal sample patterned by the same interference fringes at 30µm used for the PPLN fabrication previously described. With the term overpoling we mean that, differently from the traditional PPLN fabrication process, the external electrical field was not stopped before the poling current decrease [7,16]. The overpoling process was quite easy to accomplish . It consisted in applying one positive high voltage pulse at +12kV which was stopped when the poling current dropped to around zero, ignoring the constraints related to the current monitoring at the critical step corresponding to the lateral domain spreading. The process was about 3s long and the resulting ferroelectric domain pattern, revealed by a wet etching process of 60 minutes, is shown in Fig. 6.
Aligned dot shaped structures of sub-micrometric size are visible, originated under the photoresist strips. This effect is probably due to an incomplete merging of the adjacent reversed domains under the photoresist. In fact, single dots aligned along the photoresist fringes may be just the points excluded by the merging of the hexagonal-type counterpropagating domain walls originating from two adjacent electrodes during poling and joining under the resist lines . We believe that the propagating domain walls are not really straight because the resist strips sidewalls are affected by high corrugation and low steepness. In fact, interference fringes with pitch values over the micron scale present quite large speckles such that the propagating domain walls find ways along which the velocity of motion varies. Consequently, the merging is successful not everywhere. The optical micrograph taken in a peripheral region of the pattern and presented in Fig. 7 clearly supports this interpretation. It illustrates the domain merging occurring in different regions of the pattern such that the merging process is frozen in the various stages of its evolution. Now some questions arise. Do the sub-micrometric regions (dots) remain unreversed independently of the time duration of overpoling? Could the size of these sub-micrometric dots be designed a priori by acting on the shape of the resist pattern? Which is the minimum achievable size? Answer to these questions needs deeper investigation. We guess that the sub-micrometric relief domain structure reported by Eason and co-workers  is obtained because, in analogy with our case, the sidewalls of the photoresist strips at micron and submicron scale, achieved by masklithography, are affected by irregularities similar to that of our interference fringes.
The movie in Fig. 8 may help to understand how the mechanism of merging of adjacent domains  leads to the formation of such aligned dot domains when the poling process is performed in overpoling regime.
Examination of the etched sample revealed that opposing crystal faces displayed different structure morphologies, as can be seen in the optical micrographs shown in Fig. 9: to the point shape on the z+ face corresponds a line-shaped structure on the opposite side.
One of the interesting features of the obtained dot domains is the submicron size, as shown by the Scanning Electron Microscopic (SEM) image in Fig. 6(b). In fact, despite the advantages achieved nowadays by the conventional electric field poling technique, such as repeatability, scalability and applicability over a wide range of materials, problems remain in domain patterning at submicron scale. Such problems include non-uniform nucleation along the edges of the electrodes, uncontrolled spreading of domain walls beyond the electrode area and interaction between domain tips during forward growth. The overpoling technique presented here could be considered as an effective and relatively simple solution for achieving high density and submicron ferroelectric domain structures.
To the best of our knowledge, uniform short-pitch domain structures were reported only in two cases. Batchko et al. [14,17] obtained similar structures but by means of a completely different approach based on backswitching effect. The evidence that our domains were obtained in a different way is supported by two experimental observations. Firstly, we adopted a diode in the poling electrical circuit that prevented flowing of backswitch current. Secondly, we observed in our samples that the sub-micrometric domains were located under the mid line of the photoresist stripes whereas, according to the results reported by Batchko, the domains were produced along the electrode edges. This last observation is in agreement also with the fact the pitch of the domains, in our case, is equal to that of the photoresist pattern while in case of backswitching the pitch of domains was halved in respect to that of the electrode pattern. This happens because backswitch nucleation occurs along the electrode edges, so that the size of the insulating pattern has to be chosen taking into account the variation in the final domain size. More recently, Eason and co-workers  used overpoling. In the work presented by Eason et al., an overpoling technique is also used, resulting in the fabrication of periodically poled domain structures near the surface region but with period values limited by the resolution of the conventional photolithographic process, as discussed above.
We believe that our combination of the overpoling technique with the IL reliability in generating finer pattern periods could be considered a new approach to the fabrication of submicron-scaled inverted domains. However, the ferroelectric domain pattern obtained with our technique needs a higher homogeneity and repeatability of the achieved structure geometry to be really useful. In fact, it is possible to note that even though the lines along which the dot domains are aligned are equally spaced by about 30µm (see Fig. 6(a)), as expected, on the other hand the dots are not equally spaced along the single line (see Fig. 6(b)). This is basically due to the lack of a proper technique able to control the merging of the reversed domains under the photoresist strips during the overpoling process and to the high corrugation affecting the IL pattern when used for period values over the micron scale, as discussed previously. Further experiments about the influence of the straightness of the edges of the photoresist strips in the overpoling process are currently under investigation. One method which allows to overcome such problem consists in patterning the sample with a 2D array of insulating dots so that during the overpoling process the reversed domains are constrained to merge under the isolated photoresist structures equally spaced in both x and y direction. The results obtained by this technique are presented and discussed in the next section.
3.2 Electric field overpoling of a LN sample patterned by two-dimensional IL pattern
The electric field overpoling technique was applied to another LN crystal sample which was patterned by a 2D insulating grating consisting in a square array of photoresist dots. Such crossed grating is produced by exposing the z+ face of the spin-coated sample to the interference fringes (period of 23µm) for a set dose. This is followed by rotating the sample 90° and then exposing a second time using the same dose. Fig. 10(a) shows the optical micrograph of the resulted photoresist pattern. The overpoling technique was applied to such patterned LN sample using the electrical circuit described in the previous paragraph (see Fig. 3) and the corresponding typical current and voltage waveforms acquired during the overpoling process are presented in Fig. 11. The entire process was about 3,5s long . Fig. 10b illustrates the optical micrograph of the obtained dot domains revealed by an etching of 30 minutes at room temperature. It is clear that the 2D photoresist grating has considerably improved the 2D homogeneity of the produced dot domains.
These dot domains are equally spaced by about 20µm both in the x and y direction, in agreement with the pitch size of the printed photoresist grating, and they represent the remaining unreversed part of the ferroelectric domains after spreading under the photoresist dots. It is possible to note (see Fig. 10(b-c)) that each dot domain is surrounded by smaller dot structures which seem to describe a circle-like structure and appear similar in nature to that obtained with 1D photoresist pattern (see Fig. 6(a)). The etching process revealed that the surrounding dots have the same polarization direction as the centred ones. This suggests that the domain spreading under the dot-shaped photoresist layer evolves by leaving such small dots unreversed. In other words, the centred dots represent the regions not reached at all by domain spreading because of a too short pulse duration, whereas the surrounding ones are the result of non-homogeneous advancing of the domain wall towards the central part of the photoresist dots. The diameter of the circle-like structures generated by the surrounding dots is everywhere smaller than that of the printed photoresist dots by about 2µm from which it is possible to argue that they originated under the photoresist dots but not in correspondence of their edges. A more quantitative characterization of these surrounding dots was carried out by comparing the mean value of the ratio p/d estimated in the case of both the array photoresist layer (see Fig. 10(a)) and the etched domain pattern (see Fig. 10(b)), p being the pitch of the periodic structure in each case and d the diameter of the resist dots or of the circle-like structures. The measurements give 1.6 in the case of the photoresist layer and 1.9 in the other. That confirms the non geometrical correspondence between the surrounding dots and the edges of the resist dots.
Moreover it was observed that the z- face of the sample was flat from which it is possible to see that it has been obtained a 3D feature ferroelectric domain structure in bulk LN. In particular, it is possible to guess that the overpoling process has produced something like conically shaped domain structures developing along the z crystal axis direction without traversing the entire thickness. This morphology suggests that in LN the overpoling process evolves from the z- to the z+ face leaving these unreversed conical-like domains, as schematically shown in the movie presented in Fig. 12 which differs from that presented in Fig. 8 only for the last steps concerning the formation of the final structure.
4. Fabrication of sub-micron tip microstructures
We investigated the morphology of the obtained 3D domain patterning by using an etching process in the usual HF:HNO3=1:2 acid mixture at two different high etch-rates, taking into account that the etching process proceeds faster at elevated temperatures . Sample A has been etched at 100°C for 45 minutes and sample B has been etched at 100°C for 4 hours. Figures 13a-b show the optical micrograph taken on the z+ face of the sample A and of the sample B, respectively. It is possible to note that such etch-rates reveal the pyramidal-like morphology of the 3D domain pattern, as shown in the two SEM images taken on the z+ face of sample B and presented in Fig. 13(c-d). The white particulate visible on sample B is due to a too prolonged heat exposure in the oven, after the acid mixture had dried up.
The obtained pyramidal relief structures are highly regular in shape and quite smooth on each face, as clearly shown in Fig. 13(d). The z- face of both samples A and B is flat as schematically shown in Fig. 14, thus confirming the 3D feature observed in the previous paragraph at a lower etch-rate for the sample in Fig. 10.
These structures are similar to others presented in literature [18–20] which, however, are different in nature because of the 2D feature. In fact, the microstructuring results obtained by Barry and co-workers present samples whose opposing crystal faces have complementary structures even though with different topology. We instead obtained LN crystal structures which are 3D in nature because they emerge only from the z+ crystal face while the z- face is flat. Figure 15 presents a movie which represents a focus scanning of the pyramidal-like structures obtained on the sample A. The height of these pyramids, which depends strictly on the etching time, was estimated to be around 8µm. It is important to note that the top of these pyramids correspond to the dot domains observed on the z+ face of the 3D domain patterned sample etched at a lower rate (see Fig. 10(b)).
The submicron-scaled pyramidal-like structures obtained here by high etch-rate in 3D domain patterned LN samples could attract wide interest in the field of optics and optoelectronics, e.g. for fabricating waveguide or photonic bandgap devices.
5. Conclusions and further development
We showed that IL can be used for producing PPLN crystals and discused advantages and limitations of this alternative patterning method. IL provides a wide range of 1D and 2D pattern geometries to be used as test patterns for novel domain configurations avoiding the expensive technology of mask design and fabrication. Investigation of the overpoling process has been performed on LN crystal samples patterned by 1D and 2D IL gratings. Preliminary results show that the overpoling technique, combined with the IL reliability in making high regular patterns with periods in the submicron range, provides the possibility for domain patterning of bulk LN at a submicron scale. Such domain-engineered materials can be attractive for application to optoelectronics and integrated optical devices. Furthermore, the overpoling technique applied to 2D IL patterned samples has allowed fabrication of 3D submicron-scaled LN periodic domain patterning. Finally, we showed that a high etch-rate in these 3D domain patterns allowed us to obtain smooth pyramidal-like relief structures in the submicron scale, which could be used for fabricating waveguide or photonic bandgap devices. Current efforts are devoted to enhancing the homogeneity of the LN ferroelectric periodic domain patterning and to obtaining a quantitative characterization of the novel 3D reversed domain structures presented in this work.
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