Photolithography and EBL

The general order of the photolithography and EBL with subsequent lift-off, carried out in this work, is shown in Fig.3.3. The main steps are as follows:

Fig. 3.3: Schematic overview of a photolithography and EBL processes. Adapted from [81].

(a) Spin Coating and soft baking: after pre-cleaning with acetone and iso-propanol the substrate is covered with a thin layer of an organic polymer (resist) sensitive to ultraviolet (UV) radiation (photo resist) or electron beam (e-beam resist). The resist is put on the substrate as a drop of solution and is spun at high speeds of 1500-8000 rotations per minute (rpm), which depends on the properties of the resist and its required thickness. After spin coating, the substrate is soft baked on a hot plate at around 100^◦C for several minutes in order to remove possible solvents and stress from the resist and to improve adhesion. Parame-ters set at this point: type of the resist, spin coating speed and time, soft baking temperature and time.

(b) Exposure: the resist is patterned by exposure to UV or e-beam. In case of a positive resist, used in this work, the exposed region weakens and becomes more soluble.

In case of photolithography the patterning is done using a photomask and UV radiation. The photomask is a transparent quartz plate with an image of the desired structure patterned on one side using a non-transparent material. In case of a positive resist the target pattern on the photomask is left transparent.

Exposure is carried out by pressing the photomask against the photo resist and exposing it to a certain amount of UV radiation. Alignment of the substrate under the photomask is done using an optical microscope. As a reference point either substrate edges or previously patterned markers are used. Special trans-parent windows on the photomask are used for easier substrate alignment (see

3.1. Sample Designs and Preparation 23

Fig.3.4(a)). Parameters set at this point: type of contact between substrate and photomask (vacuum, hard or soft, depending on the pressure applied), time of exposure.

In case of EBL the patterning is done by steering a narrow focused beam of high-energy electrons (10-100 keV) across the e-beam resist on the substrate at regions, that need to be exposed. The process is often referred to as “writing a structure with EBL”. Alignment of the substrate is done using SEM and as a e-beam resist is sensitive to the electrons, edges of the substrate and/or markers are used for the alignment. Parameters set at this point: electron energy, aperture size, dose or exposure time.

(c) Wet development: the resist is developed by a solvent that depends on the resist used in previous steps. The reaction is stopped with water or isopropanol.

The development process removes the exposed part of the resist from the surface of the substrate producing a pattern that will serve as a mask for the deposited material. Parameters set at this point: development time (the rest depends on the resist).

(d) Sputtering/deposition of a material: the desired material is sputtered or deposited on top of the resist mask and the noncovered substrate surface. Pa-rameters set at this point: from the lithography perspective - the thickness, the deposition parameters that will be discussed in more detail in Section 3.1.2.

(e) Lift-off process: removal of the rest of the resist from the substrate surface.

The part of the material that was on top of the resist is removed at this point. As a result only the part of the material that was deposited directly on the surface of the substrate is left. Parameters set at this point: solvent, solvent temperature, duration length of soaking, usage of an ultrasonic bath.

Fig. 3.4: Stripes microantenna preparation overview: (a) an optical mask image, used for the resonator preparation and (b) schematic of the resonator on the substrate.

3.1. Sample Designs and Preparation 24

Photolithography

Photolithography with the use of photomasks is one of the most widely employed form of lithography in industry [82]. The main reason is a relatively fast process of a microstructure design transfer from a photomask to a photo resist. The limiting factor is resolution, i.e. the minimum feature size that can be exposed, also referred to as critical dimension. In photolithography the resolution depends on several factors such as radiation wavelength, ability of a photo resist to reconstruct the pattern, photo resist thickness, diffraction of the radiation on the edges of an image on a photomask, a gap between photomask and photo resist and its variation (tilt), the substrate reflective properties and surface quality, dirt on the photomask and/or resist, etc. The theoretical resolution R, that only takes diffraction effects into account, is given by [83]:

R= 3 2

r λ

s+z 2

, (3.1)

Fig. 3.5: Photo of the EVG 620 Mask Aligner.

where s is the gap between photomask and photoresist, λ is the wavelength of the ra-diation and z is the thickness of the pho-toresist. In contact mode, used for the sam-ple production in this work the photomask is pressed against the photoresist, which means thats= 0. considering the defects of the sub-strate, dirt and other factors, not included in the theoretical calculation, the approximate resolution of the photolithography using a vis-ible light for exposure is∼1µm.

In this thesis photolithography was carried out using an EVG 620 Mask aligner shown in Fig.3.5 and 150 mm quartz photo masks self-designed following previous research [35, 38,84] and produced by Toppan Photomasks, Inc. The main principle of the exposure process with the mask aligner was explained above when discussing Fig.3.3(b). The mask aligner is controlled by a desktop computer

EVG software. The alignment is done using a remotely controlled motorized stage while monitoring with built-in cameras. The set-up allows for different types of con-tact between photomask and photo resist, two of which were used for this thesis, soft contact for SiN-membrane substrates and hard contact for Si substrates.

In order to check if it is possible to fabricate microstrips using the available pho-tolithograpy facilities, a series of test samples was prepared. The rectangular struc-ture, patterned on the mask, with lateral dimensions l = 5µm, w = 1µm was used

3.1. Sample Designs and Preparation 25

Fig. 3.6: Test Py microstrips prepared using optical lithography with (a) AR-P 7400.23 and (b) S1813 photo resists.

(see Fig.2.2). First attempts were made with sensitive photoresist e-beam resist AR-P 7400.23 [81], which requires very short exposure and development times. The SEM image of one of the first attempts is shown in Fig.3.6(a).

The main issues are the diffraction of the light in combination with the sensitivity of the resist, as it is easily affected by diffracted light. Additionally, during exposure in contact mode a quick contamination of the photomask after only one or two uses occurs. The cause of the contamination are the dried fractions of the resist which stuck on the surface of the photomask. Therefore, the quality of the contact between photomask and photo resist gets affected leading to an increased diffraction. As a result the patterned structure gets distorted as can be seen in Fig.3.6(a) for one of the 1^st attempts.

The best attempt with the resist AR-P 7400.23 was achieved with the following pa-rameters: spin coating at 6000 rpm (∼0.5 µm thick) for 40 sec, soft baking at 85^◦C for 5 minutes, exposing for 3 sec and developing in developer AR 300-35 diluted with water in a 1:2 ratio and stopping with water. The best attempt in this case implies preserv-ing straight lines, as square corners and exact initial dimensions were not possible to obtain using this photo resist (see Fig.3.6(a) “best attempt”). The lateral dimensions of the resulting sample were l = 5.2µm, w = 1.7µm. The use of a different, less sensitive, photo resist MICROPOSIT S1805 [85] allowed more accurate dimensions of the resulting sample. The AFM image of the microstrip, produced using this resist, is shown in Fig.3.6(b). The photo resist was spin coated at 4000 rpm (∼0.5 µm thick) for 40 sec, soft baked at 115^◦C for 2 min, exposed for 5 sec and developed using MF-319 for 45 sec stopping with water. The lateral dimensions of the resulting microstrip were l = 5.1µm, w = 1.3µm. The consequence of these results was that optical lithogra-phy is not applicable for the preparation of rectangular microstrips. Nevertheless, this technique was used for the planar microresonators, as they have bigger features and allow for production errors with dimensions of up to 0.5 µm.

Designs of different types of microresonators and microantennas that were used in this thesis are shown in Fig.3.7[34,36–38,67,77,84]. Type (a) is a stripes broadband mi-croantenna (“stripes mimi-croantenna”) [35], that was used for the STXM-FMR measure-ments. Types (b) and (c) are the planar microresonators for the microresonator-based FMR measurements at ∼10 GHz (“butterfly microresonator”) [34] and ∼14 GHz

(“R-3.1. Sample Designs and Preparation 26

Fig. 3.7: Planar microantenna and microresonator designs: (a) stripes microantenna [35], (b) butterfly microresonator [34], (c) R-shape microresonator [37, 38, 67] and (d) multi-frequency microantenna [84].

shape microresonator”) [37, 38, 67], respectively. Type (d) is a broad band antenna, designed for the multifrequency FMR measurements (“multifrequency microantenna”) [84]. Both microantenna and microresonator designs consist of markers used for the alignment during the sample preparation process, a loop, where the sample is placed, and tuning elements for the measurements (see insets in the figure).

For microresonator/microantenna preparation the MICROPOSIT S1818 photo resist was employed. The resist was spin-coated at 6000 rpm (∼1.7µm) for 40 sec, soft baked at 115^◦C for 2 min, exposed for 6.5-7 seconds and developed using the MF-319 developer for 80 sec stopping with water for 1 min. The lift-off process after deposition was performed in acetone at 70^◦C for 15 min. In case of Si substrate additional an ultrasonic bath was applied in some cases. An example of the mask design and the resulting view of the substrate are shown in Fig.3.4: (a) is the photomask design for stripes microresonator with an indication of the transparent and non-transparent areas on the mask for the positive photolithography and (b) is the schematic of the substrate with the resulting microresonator structure on top of it.

Electron Beam Lithography (EBL)

Rectangular microstrips were prepared using the direct writing EBL system of an eLINE Plus from Raith. The system uses the raster scan method, i.e. the e-beam is focused into a small spot (electron probe) that is moved with respect to the substrate to expose the pattern one pixel at a time, i.e. steered [82]. The schematic of SEM is shown in Fig.3.8. The electron column and a sample are placed in a vacuum chamber in order to avoid electron scattering with anything except the sample. In case of an eLINE Plus an advanced thermal field emission is used as an electron source [86]. The energy of the emitted electrons can be varied from 0 to 30 keV.

After the electron gun emitted electrons are demagnified by first electromagnetic lens(es) (condenser) passing after that through the condenser aperture. This way an e-beam current can be controlled. The probe-forming lens is used to adjust the focus of the

3.1. Sample Designs and Preparation 27

Fig. 3.8: Simplified schematic of the EBL set-up. Adapted from [87].

electron probe and the scan coils are used for the deflection of the e-beam in order to scan across the sample [87, 88]. The distance between the sample holder and electron col-umn (working distance) has an additional ef-fect on the overall process and can also be varied. The working distance affects such pa-rameters as the depth of focus and ultimate electron probe size. For imaging and align-ment of the sample in the SEM mode the in-lens SE-detector was used. For patterning an-other changeable parameter is the dose mea-sured in 10⁻³coulombs/cm² (µC/cm²). It de-fines a minimum time to expose given area and has the following relation:

D·A=T ·I , (3.2) where T is the time to expose the area, I is the beam current, D is the dose and A is the area exposed.

For the preparation of the rectangular mi-crostrips together with the markers directly on a substrate the e-beam resist AR-P 679.02 was used. The resist was spin-coated at 4000 rpm (∼70 nm) for 40 sec and soft baked at 180^◦C for 5 min. For the pattering with EBL the following parameters were used: elec-tron energy 10 keV, aperture size 10 µm (I ≈ 0.02nA), working distance 10 mm, writing field400×400µm²and area dose 100 µC/cm².

After the exposure, the substrates were developed with AR 600-56 developer for 1 min stopping with isopropanol. After the material was sputtered the lift-off was done using AR 300-70 remover at 70^◦C for 15 minutes. An SEM image of the resulting structure is shown in Fig.3.10.

Fig. 3.10: Py microstrip produced using EBL.

For the preparation of the rectangular microstrips to-gether with the markers on a 50-100 nm thick Co:ZnO layer the e-beam resist AR-P 679.04 was used. The re-sist was spin-coated at 6000 rpm (∼270 nm) for 40 sec and soft baked at 180^◦C for 5 min. For the pattering with EBL the following parameters were used: elec-tron energy 10 keV, aperture size 10 µm (I ≈0.02nA), working distance 10 mm, writing field 400×400µm²

3.1. Sample Designs and Preparation 28

Fig. 3.9: Schematic of the main principle of the (a) magnetron sputtering and (b) PLD processes. Adapted from [89, 90].

and dose 150 µC/cm². After the exposure, the substrates were developed with AR 600-56 developer for 1 min stopping with isopropanol. After material was sputtered the lift-off was done using AR 300-70 remover at 70^◦C for 15 minutes.