Structure optimization

To optimize the design of the structure, a series of tests samples were cre- ated. Each test contains the central 25 μm of the structure (figure 5.8),

exposed to several area doses. The initial design for the Hall bar had a 300 nm constricted middle and long, 100 nm wide voltage leads, as de- scribed in section 5.2.1. Figure 5.10 shows a Scanning Electron Microscope (SEM) image of the AlOx mask after lift-off. The best area dose for this sample was 150 μC/cm2. While the central channel is clearly visible, the

voltage leads for all doses are closed. The size of the voltage leads was in- creased to 300 nm, yet still all voltage leads remained closed (figure 5.11). The width of the central channel in this design was 900 nm, however, for the optimally dosed sample we observe only 500 nm in the SEM image. We conclude that, for the optimal dose, sizes of the AlOxmask are 200 nm wider than the designed structure and we account for this in the next de- sign. The constriction in the center of the Hall bar again was open for var- ious area doses, however all voltage leads were still closed. This suggests that replacing the narrow channels of the voltage leads by a constriction might allow for easier lift-off.

(a)

Figure 5.10: a.SEM image showing the first attempt at making a side-gated Hall bar. The structure was exposed to an area dose of 150 μC/cm2. Long, 100 nm

thick voltage leads were used in the design. Lift-off of these voltage leads failed, leaving the aluminum mask intact. The middle constriction was 300 nm wide, lift-off of the middle channel was successful.

48 Results

(a)

(b)

Figure 5.11: SEM images of a follow up design for the side-gated Hall bar. aIn this design the structure was increased in size by a factor 3. The side gates are now 300 nm in the design, the main channel 900 nm. Side-gates in this structure are closed for all area doses.b.Close up of the main channel of a Hall bar (labeled 230 in panel a). The structure was exposed to an area dose of 115μC/cm2. The

middle channel is roughly 500 nm wide.

48

5.2 Side-gated Hall bars 49

A Hall bar design using constricted voltage leads was created. The voltage leads now start as 500 nm by 500 nm squares and fan out from there; the width of the central channel was changed to 700 nm. Figure 5.12 shows this pattern, created with an area dose of 145μC/cm2. The top left

piece of AlOx has been chipped, this, however, does not affect the overall structure. Notably all voltage leads for this structure are open, suggest- ing the constriction method combined with the larger size has solved our initial problems. It should be noted that all voltage leads appear different sizes. This suggests that significant thermal drift occurs while the pattern is written, leading to errors when the beam switches between the figures that make up the design.

Figure 5.12: SEM image of the first design using constricted voltage leads. The structure was exposed to an area dose of 145μC/cm2. In the design the voltage

leads were 500 nm wide and the central channel was 700 nm wide. All voltage leads in the image are open, but they are all different sizes.

To avoid thermal drift an ordering is given to the polygons in the de- sign. The pattern is written spiraling out from the center, ensuring errors are minimized between neighboring polygons. The optimal area dose has varied from sample to sample, thus we aim to overdose structures slightly, as this should lead to more consistent results. Several different voltage lead sizes were added to the design to compensate for the higher area dose. Figure 5.13 shows a structure with voltage leads of 700 nm, at an area dose of 210 μC/cm2. All voltage leads are now consistent sizes, al-

50 Results

the design next to the voltage lead, both in this sample and the previous one. To avoid this damage the corners will be more rounded off in the next design. The distance between the side-gates and the main channel is over a micron, this distance will be reduced in the next design.

Figure 5.13: SEM image of design of the Hall bar with constricted voltage leads using a writing order. The structure was exposed to an area dose of 210μC/cm2.

In the design the voltage leads were 700 nm wide, the central channel was also 700 nm wide. All voltage leads in the image are roughly 300 nm wide, the central channel is 400 nm wide.

Figure 5.14 shows a SEM image and E-beam design of the final struc- ture. Rounding off the corners has prevent damage as desired. The area dose was raised to 220 μC/cm2, the voltage leads are 700 nm wide, the

central channel is 700 nm wide and the distance to the side-gates is only 300 nm. 5.15 shows close up SEM-images of the center and voltage leads of the structure. The observed distances are summarized in table 5.1. The difference between the desired and observed distance is less than 100 nm for all parts of the design. Now that the design works we will use AFM to check quality of the lift-off.

Table 5.1:Sizes of AlOxmask in nm

Desired distance Distance in design Observed distance

Voltage Lead 100 700 160±20

Main channel 300 700 370±20

Side-gates 600 300 565±20

50

5.2 Side-gated Hall bars 51

] (a)

(b)

Figure 5.14: a. SEM image of finalized Hall bar design. The structure was exposed to an area dose of 220μC/cm2. b. E-beam design of the final structure. In this

design the voltage leads are 700 nm wide, the central channel is 700 nm wide and the distance to the side-gates is 300 nm.

52 Results

(a)

(b)

Figure 5.15:Close-up SEM images of the finalized Hall bar design. The structure was exposed to an area dose of 220μC/cm2. a. Image of the central constriction

and the side-gates. The image quality is somewhat poor due to charging effects.

b. Image of two voltage leads.

52

5.2 Side-gated Hall bars 53