Markers

The markers consist of a series of crosses, patterned throughout the sample to introduce a frame of reference for QD position mapping. The square areas that are delimited by the crosses are called marker fields (MFs) and have a side length of 50µm. All MFs can be addressed to by their label, consisting of a letter identifying the row and a numeral for the column. The grid’s origin is MF A1, located at the bottom left of the pattern. The introduced coordinate system is right-handed with the origin at the bottom left corner of MF A1.

5.2.1 Light and Dark Markers

The setup and imaging technique is developed for two types of samples with different layer compositions, i.e., as-grown samples and HBR-backed QD membranes. Initially, QD position mapping was optimized for as-grown samples that have the markers deposited on top of the optical active region, sitting on a GaAs substrate. When illuminating the sample with an IR LED, the matrix of the QDs is transparent to the light to a great extent, letting GaAs underneath absorb most of it. Markers made of gold are deposited on top of such samples, which efficiently reflect IR light, creating a good contrast in the image of light markers on dark gray background. On the other hand, HBR-backed QD membranes will reflect most of the IR light, due to the gold film at the bottom of the stack. Therefore, the marker material is changed to chromium, which is non-reflective in the IR regime, blocking most of the light reflected from the gold mirror underneath. This results in an image ofdark markers on light gray background.

5.2.2 Sample Designs

During the development of the imaging process, two designs of the distribution of markers throughout the samples are used. They differ regarding to how an EBL system detects the markers when patterning photonic structures deterministically in a later step.

The design shown in Figure 5.1 was created by C. Kohlberger for the Raith eLINE Plus in Linz, which performs linescans on marker crosses (red lines in Figure 5.1 (b)) to align the patterning system with the markers on the sample. This is repeated for every single 200 ×200µm² writefield (WF). For the system to be able to find those markers, the three red double-square structures in the corners of Figure 5.1 (a) allow a first manual alignment of the sample with the machine’s coordinate system within a so-called three-point alignment.

The whole design has a size of approximately 1.8 mm×1.8 mm and hosts 676 MFs.

In Figure 5.2, a design is shown which is suitable for an EBL machine that recognizes squares instead of lines for alignment, such as the EBL system at CNR-IFN in Rome. Auto-alignment is carried out on squares for each WF, as indicated in Figure 5.2 (b). For a first alignment, the positions of the three outer squares in Figure 5.2 (a) with respect to a point of reference on the sample holder are fed to the machine’s control software. The coordinates of these alignment markers are measured manually ex-situ, with the sample holder on a measurement stage that has a digital read-off of the x andy positions.

Figure 5.1 – (a) The marker pattern to be used for post-mapping alignment within the EBL system in Linz. The three double-square structures at the corners allow manual coarse alignment in-situ. The dashed lines separate square WFs of sizes 200 × 200 µm² each.

(b)A single WF, providing 9 marker fields. The red lines are used for automatic fine-alignment per single WF.

AA

A1 A2 A3 A4 A5 A6

B1 B2 B3 B4 B5 B6

C1 C2 C3 C4 C5 C6

D1 D2 D3 D4 D5 D6

E1 E2 E3 E4 E5 E6

F1 F2 F3 F4 F5 F6

AA AA AA AA AA

AA AA AA AA AA AA

AA AA AA AA AA AA

AA AA AA AA AA AA

AA AA AA AA AA AA

AA AA AA AA AA AA

AA

Figure 5.2– (a)The marker pattern to be used for post-mapping alignment within the EBL system in Rome. The red box shows the sample edges, the three squares allow for coarse alignment. The dashed lines separate square WFs of sizes 500×500µm²each. (b)A single WF, providing 36 marker fields. The red squares are used for automatic fine-alignment per

single WF.

Contrary to the design in Figure 5.1, MFs are not written consecutively over more WFs but are grouped into a set of 36 MFs, labeled with two letters, with the first identifying the row and the second the column. Such a set is referred to as a cell and each has its own coordinate system. There are 25 cells on one sample, allowing a total of 900 MFs that stretch close to the sample edge, which is indicated with a dark red box in Figure 5.2 (a). Each of the MFs is labeled with both the cell and MF identifier.

As a matter of fact, all samples designed for the use of the EBL in Linz were realized with light markers and those for Rome with dark markers. For this reason, designs will only be called light or dark markers from now on.

5.2.3 Fabrication of Markers

The processing steps of creating light and dark markers on a sample consist of an EBL on positive resist, evaporation of the desired marker material and a lift-off process. The fabrication sequence is sketched in Figure 5.3. Since the handling of most of those steps was already explained in detail in chapter 3, only quantitative numbers of the fabrication are mentioned here. All markers are fabricated in the cleanroom facility in Linz and by using the Raith eLINE Plus for the lithography.

First, either the3 mm×3 mmas-grown sample or the HBR-backed membrane is cleaned and put on a roughly 1 cm² large piece of Si together with other pieces using the mosaic technique explained in section 3.2.1. This ensures a homogeneous spread of the resist in the following spin coating step, where CSAR 62 is spun at 4000 rpm for1 minand cured at150°C for 5 min [59]. When patterning on membranes, local charge accumulations can form due to bad electrical contact of the surface with the sample holder, resulting in an uncontrollably deflected e-beam ruining the pattern. Therefore, the use of a protective polymer resist, Electra 92 is recommended [61]. After the underlying e-beam resist is dry and reached room temperature again, Electra 92 can be spun for 1 min at 2000 rpm and baked at 90°C for 2 min on a hot plate for a60 nmthick protective layer.

After spin coating, the resist on a neighbor piece of the mosaic is scratched with a diamond tip. Then, the sample can be loaded into the exposure chamber of the EBL system using a load-lock. When loaded, the acceleration voltage of the electron extractor is ramped up to



Figure 5.3 – Deposition of metallic markers by (a) exposure of positive-tone resist to a pattern-deflected e-beam,(b)developing the mask,(c)PVD of either Cr and Au or solely Cr and(d), releasing the resist and excess metal in a lift-off process in hot anisole, leaving only

the design sticking on the sample surface.

20 kV and an aperture diameter of10µmis set. In the vicinity of the scratch, debris can be used for imaging and alignment of important system parameters like adjusting the focus to the working distance (WD) of 7 mm, aperture position and stigmatization. Then, the sample’s position on the sample holder is matched to the system’s coordinate system by choosing the bottom left corner of the sample as origin and providing any other point on the sample’s lower horizontal edge as well, to align for the lateral tilt. Then, some debris is used for executing an iterative manual and automatic WF alignment, for ensuring a seamless stitching of WFs.

The desired patterning design can be imported as a GDS-II file and placed on the sample according to the previously defined coordinate system. To counteract a sample not lying flat on the sample holder because of the gluing, a three-point alignment is carried out, where three points outside of the desired pattern but on the sample are defined. At those points, the e-beam is exposing only a spot, i.e., it is not scanning, which will let the resist form a hill due to local melting, which can be used for focusing. For that, a small scan window with a low scan speed and a magnification of 20,000x proved useful. This offers also a possibility to double-check the astigmatization, if the hill is not symmetric. Repeating focusing on those three hills and recording the WDs within the corresponding setting completes this alignment step. The beam current is measured with the Faraday cap and is used, together with entering the desired dose for markers of 27.5µC cm⁻², by the software to recalculate the necessary dwell times, right before patterning.

300

Figure 5.4– Micrographs of(a)the mask and(b)the resulting Au markers on an as-grown sample. (d)Bent Cr markers and shifted labels, due to charge accumulation on a membrane.

(c)Sample MR77 with close-to-perfect Cr markers using a protective polymer on top of the resist. (e) The mask of MR77 before depositing Cr markers, showing (f ) only minor edge

effects on the resist, due to the improved mosaic technique.

When the writing is finished, the extraction voltage is turned off, if not happening auto-matically, and the sample is taken out. If the Electra 92 conductive polymer is applied it can removed in a1 min bath in DI-water. Afterwards, putting the sample into the alkaline devel-oper AR 600-549 for1 minand stopping the reaction in a30 sbath in isopropanol should result in a mask of high quality. The mask is carefully examined under the microscope to make sure that no charging effects are present, distorting lines of the pattern. Non-satisfactory masks should be dissolved in hot anisole so that the process can be started all over again.

Micrographs of some good-quality masks can be seen in Figure 5.4 (a), (e) and (f), while in (d) an example of Cr markers deposited on a sample showing clear charging effects can be identified by the extremely distorted marker crosses and labels.

Samples with a satisfactory mask are loaded into the Balzers - Pfeiffer PLS 570 PVD machine on "low" position, while still being on the mosaic. When the vacuum reached a low enough value, the deposition can be started. For light markers, 5 nm Cr is deposited, acting as an adhesive for the 50 nmAu layer. To fabricate dark markers, a 100 nmthick Cr layer is used. Deposition rates are 1Ås⁻¹ for Cr and 5Ås⁻¹ for Au.

The final step is a5 minlift-off in hot anisole using a sonicator. It will dissolve the hardened resist and the Cr/Au on it, as it has nothing to stick to. Only those areas where the sample was exposed to the e-beam, evaporated metal will stick in the end, i.e., one ends up with metallic markers in the patterned design. If the PMMA of the mosaic is still sticking, it can be released in acetone. The sample is then cleaned with acetone and isopropanol, blow-dried with the N-pistol and carefully examined under the microscope. Micrographs of succeeded Au and Cr markers are shown in Figure 5.4 (b) and (c), respectively, while Figure 5.4 (d) shows failed Cr markers due to the lack of a protective polymer to prevent charging effects.