Thermal Evaporation of Gold and Lift-off…

A gold thin film was deposited on the resist-patterned glass substrate by thermal evaporation. Generally, the thickness ratio between the photo resist and the metal thin film (i.e. Au) must be larger than 10:1 to ensure the lift-off process. For the PR thickness of 300-400 nm, a metal thin film under 30 nm was required. Notice that the metal film can not be too thin, namely, it should be thicker than the skin depth. The bulk resistivity of Au is 2.44μΩ-cm, so that its skin depth at 30THz is about 10nm.

Therefore a gold thin film of 30nm thickness should be suitable for our devices. For the Au deposition, the background pressure in the vacuum chamber was pumped down to 2×10⁻⁶ Torr prior to the process, and would be slightly increased to2.6−2.7×10⁻⁶ Torr during evaporation. To physically remove the unwanted residual PR in the pattern region after development, oxygen plasma treatment was engaged prior to the Au thin film deposition. For PMMA, the removal rate was about 10 nm/min, and the duration of oxygen plasma process was about 1-2 minutes without excessively decreasing the PR thickness. Au evaporation was achieved by resistive heating of the target when passing 100A current, and the deposition rate was about 0.14-0.15 nm/sec. The deposition time was controlled by a coating thickness gage, and during the process the temperature of the chamber was about 100°C.

The lift-off process was performed by soaking the specimens in the ZEP520 PR remover, ZDMAC for about 10 minutes, and then rinsed by IPA and DI water. A square was drawn around the patterned area by a tweezers prior to dipping in ZDMAC, and the specimens were gently shaken by hand during the lift-off process to accelerate the peeling of the PR. The results of the lift-off process were examined by both optical microscope and SEM. Our experiment showed that the quality of lift-off result varied according to different PR coating recipes. Figure 4-5 shows the

successful results of a lift-off process for recipe (1), in which only PR in the patterned region was removed, thus the Au layer above was lifted off. The charging effect induced by the uncovered glass can be seen evidently, this is a useful indication for a completed lift-off process. Figure 4-6 shows the results for a failed lift-off process for recipe (2). A possible reason was the residual PR in the patterned region caused by an incomplete development process, such that the Au pattern was partly peeled off during the lift-off process because of poor adhesion. Figure 4-7 to Figure 4-21 show the fifteen samples contained Au patterns that were processed using the above mentioned lift-off technique from recipe (3). Finally, Figure 4-22 is the 3D SEM image of a SRR array. The overall results were pretty good. Among thousands of patterns in each array, less than five were damaged. Some specimens were immersed in acetone and sonicated, in order to shorten the process time. This process however resulted in poorer completeness of the arrays, i.e. up to ten unit cells came off, which is a more than twice higher damage rate compared to the best one. To summary, our fabrication experience and results showed that Au thin film has adequate adhesion on glass that could be survived through the lift-off process, which thus is a feasible way to produce nano-scaled devices for bio-optical studies.

Figure 4-5 The SEM image of a SRR array after lift-off. Beam current=100 pA, field size=600 μm, PR coating recipe (1).

Figure 4-6 The SEM image of a SRR array after lift-off. Beam current=100 pA, field size=600 μm, dosage=0.9 μsec/dot, PR coating recipe (2).

Figure 4-7 The SEM image of a CRR array after lift-off. Line width=120 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-8 The SEM image of a 2-cut SRR array after lift-off. Line width=120 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-9 The SEM image of a SRR array after lift-off. Line width=120 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-10 The SEM image of a CRR array after lift-off. Line width=180 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-11 The SEM image of a 2-cut SRR array after lift-off. Line width=180 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-12 The SEM image of a SRR array after lift-off. Line width=180 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-13 The SEM image of a CRR array after lift-off. Line width=240 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-14 The SEM image of a 2-cut SRR array after lift-off. Line width=240 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-15 The SEM image of a SRR array after lift-off. Line width=240 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-16 The SEM image of a CRR array after lift-off. Line width=300 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-17 The SEM image of a 2-cut SRR array after lift-off. Line width=300 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-18 The SEM image of a SRR array after lift-off. Line width=300 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-19 The SEM image of a CRR array after lift-off. Line width=360 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-20 The SEM image of a 2-cut SRR array after lift-off. Line width=360 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-21 The SEM image of a SRR array after lift-off. Line width=360 nm, beam current=50 pA, field size=150 μm, dosage=0.85 μsec/dot, PR coating recipe (3).

Figure 4-22 The 3D SEM image of a SRR array after lift-off. Line width=360 nm, beam current=50 pA, field size=150 μm, dosage=1.2 μsec/dot, PR coating recipe (3).

Chapter 5

Measurement and Results

5.1 Measurement

Most optic measurements of μm- or nm- scaled metamaterials were done by the FTIR (Fourier transform infrared spectrometer) [30, 36, 37, 38, 39]. In this thesis work, both reflection and transmission spectra of the specimens were measured carefully using the micro scoped-FTIR Hyperion 2000 manufactured by Bruker. The measurement range of this FTIR extends from k=370-25000 cm^-1, which covers the middle-infrared (MIR), near-infrared (NIR) and visible (VIS) region by using different beam splitter and detector materials listed in Table 5-1. The wavelengths and wave numbers specifying the three spectral ranges are listed in Table 5-2. The microscope provides optical image of the sample that allows the light to impinge on the desired area when the devices area was very small, for example, 90×90 μm² arrays in our experiment. Moreover, the Hyperion 2000 equips with a grazing incidence objective to allow the light impinged obliquely at a range of 52.2°-84.2°

which is useful and convenient for reflection study. The light path of the grazing incidence objective is illustrated in Figure 5-1.

In Chapter 2, we introduced the four frequently studied orientation and polarization combinations of the SRR (Figure 2-5 that is presented here in Figure 5-2 again for convenience). Measurement geometries were designed to match these four types of combinations in Figure 5-3. Due to the mechanical limitation of our equipment,

in-plane incident is impossible. Instead, we used the grazing incidence objective to achieve horizontal quantity of the incident light and vertical quantity of the magnetic field (Figure 5-3 (a) and (b)). Polarization was set either 0° or 90° to fulfill the required direction of the electric field. Owing to the weak signal achieved by the grazing incidence objective, the measurement conditions were set differently for

Table 5-1 Beam splitter and detector materials used in Hyperion 2000 FTIR.

beam splitter detector

MIR KBr MCT/DLaTGS

NIR CaF2 InGaAs diode

VIS quartz Si Diode

Table 5-2 The wavelengths and wave numbers specifying the NIR, MIR, and VIS spectral ranges. Most of the present spectral analysis are located in the range k=4000-400cm^-1, i.e., the MIR region.

λ(μm) k(cm^-1) MIR 2.5-50 4000-200 NIR 0.78-2.5 12800-4000 VIS 0.4-0.7 25000-14300

Figure 5-1 The light path of the grazing incidence objective in Hyperion 2000.

Table 5-3 Measurement conditions vertical incidence oblique incidence aperture 1.5 (mm) 8 (mm)

resolution 4 (cm^-1) 8 (cm^-1) scan time 32 (time) 64 (time)

Figure 5-2 SRR in the four nontrivial EM field propagation directions and polarizations. The figure is imaged from [36].

Figure 5-3 Measurement geometries that correspond to the four combinations in Figure 5-2, accordingly. Reflection was measured through (a)-(d), while transmission was measured only for (c) and (d).

oblique and vertical incidence as listed in Table 5-3. Despite the aperture set, the measurement window was narrowed down to approximately the array size, i.e. 90 μm, for both oblique and vertical incidence. Transmission measurements were done only for geometries (c) and (d) since it is not available for oblique incidence using our equipment.

5.2 Analysis procedure

Because the intensity of the incident light varies with the impinging and polarization angles, there is a need to calibrate and normalize the measured signal intensity, in order to facilitate a unified comparison among various spectra. This was done by drawing a slope from the lowest point to the highest point for each spectrum.

The lower intensity then can be compared to the higher intensity with a meaningful reference.

To highlight the spectral features that are caused by interactions between the incident light and the patterns, the measured spectrum were divided by the background spectrum. The background of the reflectance was a gold-coated reference specimen, while the background of the transmittance was the glass located outside the patterned region. The reflection/ transmission spectra that have been normalized and divided by the proper background spectra are called normalized reflectance/

transmittance.

Expected EM responses for the three types of ring resonator patterns under different measurement geometries are listed in Table 5-4. The main factor that influences the response of the resonators is whether the magnetic field is perpendicular or parallel to the pattern surface. Another factor is the orientation of the pattern, i.e. the gap-bearing side of the resonators, with respect to the electric field.

Table 5-4 Expected responses for the three types of ring resonators, i.e., SRR, CRR, and 2-cut SRR, under the four measurement geometries depicted in Figure 5-2 and Figure 5-3. E stands for the electric resonance, M stands for the magnetic resonance and C stands for the electric coupling effect.

(a) (b) (c) (d)

SRR E/M E/M/C E E/C

CRR E E E E

2-cut SRR E/M E/M E E

5.3 Results and Discussion

The measurements were essentially carried out twice for each of the three samples, and the obtained spectra are rather identical, indicating a good experimental reproducibility. Figure 5-4 shows the normalized transmittance of the SRR array for geometry (c). The transmission dips correspond to the peaks of the normalized reflectance for the same SRR array and measurement geometry shown in Figure 5-7, while strong absorption occurred for frequency below about 60 THz. This might be a transmission property of the Conning glass wafer, namely, the glass is opaque for part of the MIR region. The expected magnetic resonance wavelength 6-18 μm, i.e. 16-50 THz in frequency, was blocked by this substrate absorption. Thus, we will present only the normalized reflectance spectra in the following discussion.

0 5 0 100 1 50 200 2 50

Figure 5-4 Normalized transmittance of the SRR array for geometry (c).

Figure 5-5 to 5-8 show the normalized reflectance of the SRR array for the four measurement geometries. For all measurement geometries, there is a reflection band from 24 to 36 THz, while at higher frequency, another reflection peak can be clearly seen and has a blue shift as the unit cell size decreases. Some of the reflection peaks split especially for small unit cell size (600nm and 900nm) and for oblique incidence ((a) and (b)). This might be due to the incapability of the machine to cover the entire spectral region smoothly. The orientation of the electric field influences the reflection spectra by changing the spectral configuration which can be seen when comparing (a) and (b) or (c) and (d). When the electric field was parallel to the gap-bearing side of the SRR, the reflection peak shifted to higher value.

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Figure 5-5 Normalized reflectance of the SRR array for geometry (a).

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Figure 5-6 Normalized reflectance of the SRR array for geometry (b).

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Figure 5-7 Normalized reflectance of the SRR array for geometry (c).

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Figure 5-8 Normalized reflectance of the SRR array for geometry (d).

Figure 5-9 to 5-12 show the normalized reflectance of the CRR array for the four measurement geometries. Again, the reflection band between 24-36 THz is fixed in all measurement geometries. The reflection spectra of the CRR array are quite similar to that of the SRR array for geometry (a) and (c), while there is almost no difference between (a) and (b) or (c) and (d) which implies that the orientation of the electric field does not affect the EM response of the CRR array. This is reasonable and expected since the CRR is symmetric. Because the CRR does not have a capacitance, it is assumed to exhibit only the electric response. And since the spectrum of the CRR array for geometry (a) does not remove any peak when comparing with that of the SRR array, one may conclude that the reflection peaks occur beyond 50 THz should be an electric response rather than a magnetic one.

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Figure 5-9 Normalized reflectance of the CRR array for geometry (a).

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Figure 5-10 Normalized reflectance of the CRR array for geometry (b).

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Figure 5-11 Normalized reflectance of the CRR array for geometry (c).

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Figure 5-12 Normalized reflectance of the CRR array for geometry (d).

Figure 5-13 to 5-16 show the normalized reflectance of the 2-cut SRR array for the four measurement geometries. The fixed 24-36 THz reflection band is consistent with the previous SRR and CRR case. We may conclude that this unwanted reflection band is due to the feature of the glass substrate. The spectra for geometry (a) and (c) are also similar to that of the SRR and CRR, while in (b) and (d), the spectra vary in a different way from that of the SRR. In a nut shell, the three patterns show finger print spectra result for geometry (b) and (d).

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Figure 5-13 Normalized reflectance of the 2-cut SRR array for geometry (a).

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Figure 5-14 Normalized reflectance of the 2-cut SRR array for geometry (b).

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Figure 5-15 Normalized reflectance of the 2-cut SRR array for geometry (c).

0 50 100 150 200 2 50

Figure 5-16 Normalized reflectance of the 2-cut SRR array for geometry (d).

To examine the scaling of the EM response with the unit cell size, the electric resonance wavelength (and its corresponding frequency and wave number) of the five SRR unit cell sizes measured from geometry (c) are listed in Table 5-5 and the λp–a relation is plotted in Figure 5-17 which shows a nearly linear scaling. Here we are unable to draw the scaling of the magnetic response since it was not observed in our measurement results.

Table 5-5 The numeral data of the electric response wavelength and corresponding frequency and wave number for different unit cell sizes.

a (nm) λp (μm) fp(THz) kp (cm^-1) 600 2.37635 126.24383 4208.12764 900 3.33239 90.02539 3000.84629 1200 4.17825 71.80045 2393.34849

Figure 5-17 The scaling of the electric response wavelength with unit cell size for the SRR array measured by geometry (c).

To reveal the influence of the electric field orientation on the reflection spectrum, the measured spectra of geometry (a) and (b) were plotted in the same graph, while those of (c) and (d) were plotted in another one, i.e., with the same polarization but different beam impinging azimuths for comparison. Figure 5-18 summarizes the spectra of the three patterns for the smallest unit cell size, 600 nm. It can be clearly seen that the spectra of the CRR were not affected by the E orientation, while for SRR and 2-cut SRR, the electric resonance peaks shift to higher value in geometry (b) and (d). The degree of shift is larger for the SRR than that for the 2-cut SRR. This is expected since the 2-cut SRR is more symmetric than the SRR (but not as symmetric as the 4-cut SRR), it should be able to reduce, if not remove, the electric coupling effect.

Namely, the electric resonance peaks are less influenced by the E orientation for more symmetric ring resonators. However, the reflection peak near the magnetic resonance frequency induced by the electric coupling effect was not observed in our measurement results except for the smallest 2-cut SRR where an additional reflection peak at about 60 THz occurred as shown in Figure 5-18 (F). One possible reason may be that the magnetic response frequency is smaller than expected, so only the peak of the smallest 600nm 2-cut SRR (which has smaller capacitance and thus higher magnetic response frequency then the 600 nm SRR) can survive the blocked region of the spectra. For the 900 nm 2-cut SRR, a secondary reflection peak is also observable at about 46 THz as shown in Figure 5-19.

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Figure 5-18 Comparison of the normalized reflectance for the (A) SRR arrays in geometry (a) and (b), (B) SRR arrays in geometry (c) and (d), (C) CRR arrays in geometry (a) and (b), (D) CRR arrays in geometry (c) and (d), (E) 2-cut SRR arrays in geometry (a) and (b), and (F) 2-cut SRR arrays in geometry (c) and (d). The unit cell size is 600nm.

0 50 100 150 200 250 0

2 4 6 8 10 12

CUT 180

Normalized reflectance (a.u.)

Frequency (THz)

c d

Figure 5-19 Comparison of the normalized reflectance for the 2-cut SRR arrays in geometry (c) and (d). The unit cell size is 900nm.

Chapter 6

Conclusion

Meta-materials were studied in this thesis work because of their resonance features.

Three types of ring resonators: SRR, CRR, and 2-cut SRR were designed with five unit cell sizes, ranging from 600 nm to 1800 nm. The success in fabrication of Au pattern arrays indicates the feasibility to make nano-scaled gold devices on the glass substrate using the e-beam lithography together with the lift-off technique.

Optical measurements were carried out using the microscoped-FTIR for four nontrivial geometries. The magnetic resonance was not observed due to the overlapping of the interested frequency region with the substrate absorption, i.e. Si-O bond vibration, around 10 μm. This implies that glass may not be a suitable substrate for optical studies at the MIR region.

However, the electric resonance of these patterns showed clear trends with reproducibility. First, the electric resonance frequency was influenced by the orientation of the resonators with respect to the electric field. When the electric field is parallel to the non-gap-bearing sides of the resonators, the reflection spectra were similar for SRR, CRR, and 2-cut SRR. In contrast, when the electric field is parallel to the gap-bearing sides of the resonators, the reflection spectra of the three types of the resonators became different. The reflection spectra of the CRR remained almost unchanged while the electric resonance frequency shifted to much higher values for the SRR (blue shift). For 2-cut SRR, only slight shift of the peaks were observed

which implied that more symmetric resonators can effectively reduce the electric coupling effect. On the other hand, a linear scaling of the electric resonance with the unit cell size has been observed.

In order to demonstrate both electric and magnetic response of the ring resonators, it is useful to utilize different ring resonator designs with various unit cell sizes and measure the optical features for all possible geometries that induce the EM response and cross-check the spectra.

To study the magnetic resonance of the ring resonators at 10 μm, either the glass substrate should be replaced or the unit cell size should be tuned. Simulations prior to the experiment by CST Microwave Studio, a state-of-the-art electromagnetic field solver [1, 2, 25, 37, 43], will provide useful information and physic insights that may help and allow more reasonable design of the device. In the future, to adhere

To study the magnetic resonance of the ring resonators at 10 μm, either the glass substrate should be replaced or the unit cell size should be tuned. Simulations prior to the experiment by CST Microwave Studio, a state-of-the-art electromagnetic field solver [1, 2, 25, 37, 43], will provide useful information and physic insights that may help and allow more reasonable design of the device. In the future, to adhere