Periodically-Patterned Etch Features

Periodic patterning of the etch-frustrated features was investigated via illumination through a +1/−1 order phase mask with a period of Λ = 726 nm and designed for a wavelength of λ = 246 nm, as detailed above in Section 4.1. Periodic illumination with fluences below the ablation threshold produced etch dots similar to the case of un- patterned illumination, however it provided control over the size, spacing, and alignment of these etch dots, as shown under several magnifications in Figure 4.6. Variations in the periodic pattern shows that the formation of these features are highly sensitive to the local illumination conditions and material properties.

The images of Figure 4.6 show that the frustrated etch features reproduce the phase mask pattern along the axis of periodicity. However using a phase mask of this type,

an intensity pattern with half the period of the phase mask (363 nm) was expected. The consistent appearance of features on a scale equal to the period of the phase mask indicate that the diffracted 0-order beam could not be neglected. This may be due to the combination of the non-optimal wavelength (248 nm), a small angle away from normal incidence, and low quality of the phase mask. Nonetheless, regions exhibiting a period of 363 nm have also been observed [Mailis05b], although this occurred much less frequently.

Perpendicular to the phase mask grating vector, the etched feature sizes were not directly controlled in a periodic manner. Nonetheless the lengths of these features were similar to their widths. It is likely that through two-dimensional periodic patterning (such as through the use of a two-dimensionally periodic phase mask), the size of these features may be controlled in both spatial directions.

The effect of etch frustration is highly sensitive to incident fluence, as seen in Fig- ure 4.6(a), where variations across the illumination region caused imperfections in the periodic pattern. Surprisingly, increasing the fluence towards the ablation threshold caused the surface to become smoother, as depicted in Figure 4.7. With a fluence of ∼340 mJ/cm2_{, the etched dots began to merge along the periodic intensity lines (a).}

A fluence of ∼370 mJ/cm2 began to show merging of the peaks of these etch dots (b). Increasing the fluence further allowed a greater merger between adjacent etch dots (c–e) until merging occurred between adjacent lines (f). Counter-intuitively, illumination by fluences approaching the ablation threshold caused the merger of dots, forming a smooth surface after etching.

A similar trend has recently been observed by Sakellaris Mailis and Alistair Muir (Uni- versity of Southampton) in unpublished work investigating the interaction of scanned cw UV light of λ = 244 nm from a frequency-doubled Ar+ laser. Increasing the in- tensity or dwell-time of the beam altered the scanned region from a rounded line [as in Figure 4.7(a)] to a straight line with a smooth, fully-connected top surface [as in Figure 4.7(c–d)].

A possible explanation for the above trend is UV-induced domain inversion. If domain inversion had occurred, a +z domain face would be presented, resisting etching and hence providing the smooth surface observed. The validity of this proposal is evaluated in the following sections.

4.2.4 Scanning Force Microscopy

Scanning force microscopy (SFM) has become a powerful tool for non-destructive visual- ization and manipulation of domain engineered materials. Therefore, this technique was applied to the crystals having either unpatterned or periodically-patterned illumination

(b)

(a)

(c)

(d)

(e)

(f)

1 mm

1 mm

1 mm

1 mm

1 mm

2 mm

Figure 4.7: SEM micrographs showing progressively more merging of UV-induced

features formed via 10-pulse illumination withλ= 248 nm light through a phase mask of Λ = 726 nm, by increasing the fluence of each spot: (a) 340, (b) 370, (c) 400, (d) 430, (e) 460, (f) 490 mJ/cm2_{. The insets show the same illumination conditions at lower}

magnification.

with fluences below the ablation threshold. These crystals were not HF-etched after illumination as this process may destroy the features intended for further visualization.

Access to SFM facilities and expertise was made available by collaboration with Professor Karsten Buse’s group at the University of Bonn, Germany, and Associate Professor Venkatraman Gopalan’s group at Pennsylvania State University, USA. At each of these locations, a custom-modified AFM was fitted with an electrically-conductive tip through which oscillating voltages applied large electric fields locally over the surface of a sample under test. A lock-in amplifier supplied the voltage, typically Vapplied = 10 V peak-to-

Figure 4.8: The−z face of undoped CLN exposed to 25 pulses of 248 nm UV light

via a phase mask of period 726 nm with a fluence∼70 mJ/cm2_{, viewed by (a) SEM}

after HF etching, and (b) SFM amplitude scan without etching.

cantilevers with large spring constants (∼14 N/m) were used to limit the interference from surface electrostatic effects [Hu02].

4.2.4.1 SFM of UV-induced Features

The −z face of undoped CLN was illuminated by 25 pulses of λ = 248 nm light with a fluence of ∼70 mJ/cm2, and patterned by a phase mask with Λ = 726 nm. After HF etching, etch dots formed along the periodic intensity lines with feature sizes down to 100–200 nm, as shown in Figure 4.8(a). A similarly exposed, unetched sample was investigated by SFM imaging, shown in Figure 4.8(b). These images show a clear one- to-one correspondence between etch features and PFM amplitude response.

In addition to imaging, the SFM is also capable of applying large electric fields suitable for inverting ferroelectric domains. A large voltage of ∼100 V DC was applied by the group at the University of Bonn to UV-exposed crystals. Figure 4.9 shows the SFM image both (a) before and (b) after the high-voltage DC-EFM scan across a portion of the irradiated region, delineated by the dashed lines. Within this high-voltage scan area, the contrast induced by the periodic UV light had been erased. These results imply that the contrast observed by SFM imaging may be the result of ferroelectric domains which have been re-poled through the application of higher tip voltages.

Recently, the cw UV scanned-beam experiments of Sakellaris Mailis and Alistair Muir (University of Southampton), introduced in Section 4.2.3, have also been investigated by PFM by Elizabeth Soergel (University of Bonn). In this instance, cleardomain contrast

has been observed for lines formed in the regime of high-exposure, indicating shallow surface domain formation.

Figure 4.9: SFM scans showing the same region of undoped CLN exposed to UV

illumination via a phase mask (a) before and (b) after erasure in the central region enclosed within the box by applying 100 V DC to the tip. The large bright and dark regions within the scan were believed to result from tip degradation which occurs on

the application of high voltages.

4.2.4.2 SFM at the Surface

SFM scans of UV-induced features have produced contrast in amplitude-response only. To date, very little contrast has been observed in any of the phase-response scans. To understand this lack of contrast, the depth of these features must be considered.

For bulk domains, SFM is capable of clearly showing 180◦contrast between up and down domains, as evidenced from both the literature and SFM scans completed at the facilities of Pennsylvania State University. In this case, the piezoelectric signal of one domain is in phase with the applied electric field, while the signal from the opposite domain is precisely 180◦ out of phase with the applied electric field. For non-bulk domains, where the crystal can be viewed as a composite material of two opposite domains stacked one on top of the other [Johnston03], one must recall that the electric field emanating from the SFM tip diminishes with increasing distance, yielding an effective penetration depth into the crystal. There are two main cases to consider for these surface domains.

For deep surface domains where the effective penetration depth of the electric field is comparable to or less than the surface domain depth, the material response is dominated by the surface domain. Therefore, the piezoelectric AC signal from the surface domain has a large amplitude, A, and a phase shift, φ, while the signal from the domain below (i.e. the bulk) has a smaller amplitude, a, and a phase shift, (φ+π). Therefore, the sum of these contributions to the measured signal is,

resulting in a signal with the same phase as that of the surface domain, but a diminished amplitude. For such a deep surface domain, contrast in both amplitude and phase can be measured relative to the surrounding virgin crystal.

In the second case, for shallow surface domains where the effective penetration depth of the electric field from the SFM tip is much greater than the surface domain depth, the material response is dominated by the bulk below the surface domain. In this case, the piezoelectric AC signal from the surface domain has a small amplitude, b, and a phase shift, φ, while the signal from the bulk has a larger amplitude, B, and a phase shift, (φ+π). Therefore, the sum of these contributions to the measured signal is,

dshallow_p =bsin(φ)+Bsin(φ+π) =−bsin(φ+π)+Bsin(φ+π) = (B−b) sin(φ+π) (4.2)

resulting in a signal 180◦ out of phase with the surface domain and a diminished am- plitude. For such a shallow surface domain, contrast is observed only in the amplitude signal because the total phase is the same as that for the surrounding virgin crystal.

In reality the field is of course three-dimensional, thus requiring consideration of both lateral dimensions as well, meaning measurements of surface domains small in lateral extent will have even more interference from the crystal environment than in the simple one-dimensional discussion above. Nonetheless, this model is capable of explaining why the SFM scans of the UV-induced surface features display contrast in the amplitude signal only, falling into the case of the shallow surface domains.