Scanning Electron Microscope

SEM is an electron microscope that produces the picture by bombarding the sam-ple surface with high energy electrons to excite the surface atoms. These atoms emit secondary electrons, which are collected by a detector to produce an image of

the surface. This is done by comparing the location and intensity of the secondary electrons. The working principle of SEM is similar to the EBL, the electrons are accelerated towards the sample and controlled by electromagnetic lenses. The dif-ference is mainly on the acceleration voltage used, as it is much lower in the case of SEM. The basic layout of an SEM system can be seen in Fig. 3.2. [14,31]

Electron source

Anode

Condenser lens

Scan coils

Objective lens

Sample Sample stage

Detector

Figure 3.2: A basic schematic of an SEM system.

The SEM device used during this work was SEM Leo 1550 Gemini. As venting the system takes a considerable amount of time, the venting process was started while the samples were placed onto a cross-section sample holder. The holder we used can hold two samples simultaneously to image the cross-sections of the samples.

After the samples were on the holder, nitrogen gas was blown over the samples to reduce the number of particles on the samples and the holder. After the venting process, the chamber door was opened. The holder was slid to this attachment until the holder is secure inside the system. Then the chamber door was shut and the

‘Exchange’ button pressed again to start the evacuation process.

While the system was evacuating, the sample was moved near the electron gun

with the controls of the system. This was to mitigate the downtime due to the time the system takes to evacuate. The acceleration voltage was then started by pressing the ‘EHT’ button and selecting ‘EHT On’. Now, the system camera was from the CCD sensor used for roughly positioning the sample under the detector, to ‘Inlens’

which is the secondary electron detector [31].

Brightness and contrast were changed to have a clear image. This was done by setting brightness to 50 % and then tuning the contrast until the image was clear. Also, the gun alignment and aperture alignment were checked. If these had reasonable values, the scanning speed was set to the fastest, and then the stage was moved to find the samples in the SEM image. When the sample was found, the marks that were made earlier were searched from the image, while changing the focus and magnification with the controls of the system when moving between different areas of the sample.

When the grating was found, we tried to focus on it. However, in most cases, due to stigmation, a clear picture could not be found at first. When this occurred, we proceeded to find a particle or other imperfection on the cross-section surface.

Most of the time it was not hard to find these. Next, we proceeded to zoom into the particle and lower the scanning speed while using a reduced aperture around it.

Then we iterated between the focus, stigmation X, and stigmation Y and tried to decrease the stigmatization until the particle was in full focus. If this particle was in the same magnitude as the grating, the scan area was moved to the grating area and only minor adjustments are needed to get a clear image. Otherwise, one might need to do these iteration cycles a couple of times.

For the taken images, the magnification was set to 200 000x and when the image was focused, the scan speed was lowered and the image was frozen. The magnifi-cation was held the same for all of the measurements. From the frozen image the periodicity and the height of the structure were measured. Also, in the cases where there were still some resist on top of the silicon, the height was measured both to the top of the silicon structure and the top of the whole structure with the resist.

This was done to make selectivity measurements possible.

Chapter IV

Results

In this chapter, the results of this work are displayed and their implications discussed.

As this work discusses the etching process of an analogous structure, namely a blazed grating, the process needs not only to etch deep enough to the substrate but also to preserve the shape of the structure as well as possible.

The performance of the etching process needs to be evaluated by some means.

Generally, these parameters are etching rate, selectivity, uniformity, surface quality, reproducibility, residue, microloading effects, and so forth [12]. In other words, there is a myriad of ways to describe the performance of an etching process. For this work, we are mainly interested in the selectivity and the etching rate of the process.

We also would like to see, if this process is reproducible. We were interested in surface quality and uniformity, as these are also important for the process. Pattern preservation is an important parameter in the case of analogous structures, as the pattern should not deform during the etching process. In the case of selectivity, the problem mostly is that it is difficult to compare two different analogous structures with just the selectivity parameter. Therefore, it makes the selectivity a complicated parameter to use in our case and makes it a sufficient comparison method only when the shape is similar between the samples. Thus, the process should be characterized in two parts. Firstly, one should see that the shape of the structure is sufficiently preserved. Then, the rest of the parameters should be at least comparable to each other.

The process started with an initial guess, and then proceeded with coarse adjust-ment of parameters and then comparing the results. The initial parameters were:

a gas flow of SF6 and O2 were 25 and 10 standard cubic centimeters per minute

(sccm) respectively, chamber pressure of 10 mTorr, RF power was set to 200 W and the sample was etched for one minute.

4.1 Gas composition

The gas composition test was the first one to be done in this study. The gas compo-sition was noticed to have the most significant effect on the shape of the structure.

In this process, there is SF₆ and O₂ gases in the chamber simultaneously. These gases etch different materials during the process. Oxygen ions do not etch silicon but do etch the resist layer on top of it, while SF₆ etches the silicon. We decided to test around the initial guess, first etching with a higher percentage of SF₆ gas than on the initial guess and then with a higher percentage of O₂. The parameters used can be seen in Table 4.1.

Table 4.1

Parameters of SF6and O2 for each gas composition test.

Sample A Sample B Sample C

SF₆ (sccm) 25 30 20

O₂ (sccm) 10 5 15

When we compare the results of these gas composition tests, we can see from Fig.

4.1 that there is a vast difference in shape when the gas composition is altered. In Fig.4.1(a), from now on called sample A, one can see that the shape of the structure is far from ideal. The shape is somewhere in between of binary and blazed gratings.

There is still some resist left on top of the silicon. The thickness of the resist on top of the structure is 34 nm in this case. This transition from silicon to resist can be seen when the edges of the shape suddenly have an incision. As there is some resist on top of the structure, we should increase the percentage of oxygen in the process to increase the etching rate of resist.

In the case of Fig. 4.1 (b), sample B, the etching process is too directional and, therefore, cannot keep the blazed shape. The shape of the structure resembles a bi-nary structure with triangular tops. There is also some resist on top of the structure, which can be seen as darker triangles on top of the structure. The thickness of the resist on top of this structure is 68 nm. The thickness of the resist layer increased

from the earlier structure as the amount of oxygen in the process were lowered. As the percentages of SF₆ is higher, the anisotropicity of the process is increased.

(a) (b)

(c)

Figure 4.1: The results of gas composition modification tests. Figure (a) is from the Sample A, figure (b) is Sample B and figure (c) is the Sample C from the Table4.1.

In the case of Fig. 4.1 (c), sample C, we can see that the resist is practically etched away from the structure surface. The shape of the structure is preserved in this process, as it starts to have a triangular shape. This shape is, nonetheless still not good enough and the recipe needs to be modified further. However, the structure is so close, that altering the gas composition here could cause too large shifts in the shape of the structure. Therefore, this gas composition was chosen for further modification of the recipe, as the other parameters would not alter the shape as significantly as the gas composition. This gas composition also gets rid of almost

all of the resist on top of the structure with the etching time of one minute. There are still some resist left on top of the structure. This also needs to be etched away to finish up the process.