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Electrostatic electron mirror in SEM for simultaneous imaging of top and bottom surfaces of a sample

The use of electron mirrors in aberration correction and surface-sensitive microscopy techniques such as low-energy electron microscopy has been established. However, in this work, by implementing an easy to construct, fully electrostatic electron mirror system under a sample in a conventional scanning electron microscope (SEM), we present a new imaging scheme which allows us to form scanned images of the top and bottom surfaces of the sample simultaneously. We believe that this imaging scheme could be of great value to the field of in-situ SEM which has been limited to observation of dynamic changes such as crack propagation and other surface phenomena on one side of samples at a time. We analyze the image properties when using a flat versus a concave electron mirror system and discuss two different regimes of operation. In addition to in-situ SEM, we foresee that our imaging scheme could open up avenues toward spherical aberration correction by the use of electron mirrors in SEMs without the need for complex beam separators.


Introduction
In its simplest form, an electron mirror creates an electric field in which incident electrons slow down to a complete standstill before being accelerated away in the opposite direction. Multi-electrode electron mirrors have been used in mirror electron microscopy (MEM) [1,2,3,4,5] and low-energy electron microscopy (LEEM) [6,7,8,9]. Today, aside from LEEM, the most prominent application of electron mirrors is in chromatic and spherical aberration correction [10,11,12]. More recently, designs for novel electron microscopy schemes that lower beam-induced damage to fragile biological samples have been proposed. Under the banner of quantum electron microscopy (QEM), two such designs have been proposed that are based on electron cavities consisting of two electron mirrors between which electrons would reflect back and forth to probe a sample multiple times [13,14,15].
Implementation of electron mirrors in SEMs has not been widely reported.
In this work, we explore the use of tunable, easy to construct electron mirror systems in an SEM under different imaging regimes.
During the development of electron mirrors for the QEM project, we incorporated a fully electrostatic multi-electrode mirror in a conventional SEM, and found a new imaging scheme in which it is possible to produce simultaneous scanned images of top and bottom surfaces of perforated samples.
As shown in Fig. 1(a) a focused electron beam scans a perforated sample, e.g. a transmission electron microscope (TEM) copper mesh. Where the sample obstructs this scanning probe, a scanned image of the top surface of the sample is produced. At scan positions where the probe is not obstructed, the electrons pass through the sample plane towards the electron mirror system placed below. This electron mirror system which consists of a mirror electrode and three electrodes to form a lens is commonly referred to as a tetrode mirror and in this case reflects and refocuses the beam back on the sample plane. Where this reflected scanning probe strikes the bottom surface of the sample, a scanned image of the bottom surface of the sample is produced in the same micrograph. This is due to the fact that the greyscale value of each pixel in an SEM micrograph is dependent only on the number of secondary electrons detected while the beam was dwelling over that pixel as opposed to the source of those secondary electrons. The end result is an SEM micrograph in which the top and bottom surfaces of the sample are simultaneously visible as shown in Fig. 1(b). It must be noted that we employ the term "simultaneous" loosely here; depending on the scan speed, there is microseconds to milliseconds between when the top and the bottom images form. In this work, we refer to the image of the top surface as the direct image and the image of the bottom surface as the reflected image.
A similar imaging technique in a different context was demonstrated by Crewe et al. using custom made scan coils and a magnetic lens [16]. In our approach, we take advantage of a fully electrostatic tetrode mirror, which could be easily retrofitted in most conventional SEMs. Furthermore, since the operation of our mirror does not rely on a magnetic immersion lens, the sample could be held at a nominally field-free region which is of interest for investigation of magnetic samples.
The most immediate application of our imaging technique is its potential use in in-situ SEM where it is of interest to image dynamic changes such as crack propagation or melting of a sample [17,18]. Currently, in-situ SEM is limited to capturing changes on one side of the sample at a time. The multielectrode mirror and the imaging scheme described in this work could be  used to simultaneously capture dynamic changes on both surfaces of a thin sample. Incorporation of this mirror system in an SEM chamber could be easily achieved due to its relatively small form factor and ease of assembly and operation. For achieving the highest possible resolution, careful design, machining and assembly of multi-electrode electron mirrors are crucial.
We demonstrate that even without meeting those stringent requirements, a simple electron mirror system constructed with off-the-shelf components is nonetheless capable of reflecting the beam and forming a scanning focused probe of about 100 nm diameter to produce reflected images.
Another potential application for our imaging scheme is mirror aberration correction. Conventional mirror aberration correction has relied on separation of the incident (uncorrected) and the reflected (corrected) beams by means of either Y-separators [10,11] or 90-degree separators [19]. These separators often introduce chromatic dispersions of their own to the beam.
Although later designs, notably one by R. Tromp [12] eliminated these chromatic dispersions, such beam separators remain complex and are generally not designed for use in SEMs. More recently, Dohi and Kruit showed a promising design in which two miniaturized electron mirrors along with smallangle deflectors are used in order to correct the aberrations of an SEM ob-jective lens [20]. However, their approach requires significant modifications to the electron column.
Our imaging scheme could lead to a fundamentally different approach to mirror aberration correction by eliminating the need for deflectors of any kind. In a similar imaging setup as shown in Fig. 1(a), one could replace the flat mirror electrode (V1) with an aperture electrode in order to create a tunable curved mirror potential. With the mirror voltages tuned correctly, one could image the bottom surface of a perforated sample with a spherical-and/or chromatic-aberration-corrected probe without the need for beam separators. This simple approach comes with a number of limitations chief among which is the need for samples that do not block the entire field of view. It must be noted that although we demonstrate image resolution improvements when using a concave five-electrode mirror system, we do not attempt rigorous aberration correction which is an involved process and hence beyond the scope of this work. In this work, as shown in Fig. 1(a), we refer to the mirror system electrodes, as V1 through V4 from bottom to top. Electrode V1 is a flat stainless steel plate to which a negative bias larger in magnitude than that of the acceleration voltage of the beam is applied. Electrode V2, sometimes referred to as the cap electrode, is an electrode with an aperture at its center. It acts as a field limiting aperture for the mirror electric field and due to the curved field lines near its aperture, effectively behaves as a diverging lens [5,21].

Experimental design
For simplicity, we keep this electrode grounded. The role of electrode V3 is to provide a positive lensing effect, allowing for refocusing of the reflected electron beam onto the bottom surface of the sample. Hence, we refer to this electrode as the lens electrode. Finally, the grounded electrode V4 completes the lens component of the tetrode mirror and minimizes the amount of stray fields from the electrodes below.

Mechanical considerations
The form factor of the electron mirror system is dictated by the available space inside the SEM, mechanical and machining constraints, as well as electric field and electron optical requirements. With these considerations, we opted to use commercially available stainless steel plates with regularly spaced through-holes in order to construct the mirror stack.   [23]; however, this requires the use of micromachined optics which we do not pursue in this work, as we focus here on an "off-the-shelf" approach.

Results and discussion
In this section, we discuss imaging under different electron optical regimes defined by the trajectory of the reflected beam as determined by the strength of the lens element within the mirror system. Furthermore, we present reflected images produced with a curved mirror constructed with five electrodes. We analyze image properties associated with each regime and experimental setup.

Broad beam over the mirror surface
The first regime of operation is defined by the tetrode mirror voltages set such that for a given beam energy, the back-focal plane of the mirror and lens coincides with the sample plane. An incoming electron beam that is focused on the sample plane by the objective lens of the SEM will enter the tetrode mirror and consequently be spread out over the mirror electrode.
After reflection, the beam will be refocused on the sample plane. Figure   3(a) shows a schematic of the electron trajectory and Figure 3  Since the incident beam is spread out over the mirror plane, this regime of operation is also useful for applications in which manipulations to the electron wave front using an electron mirror is of interest. An example of such application is electron diffraction by means of a diffractive electron mirror [24] which is of interest in novel low-damage electron microscopy schemes [13,25].
Another example of an application in which this regime of operation is used is mirror aberration correction which will be discussed in Section 3.3.  Fig. 3(d).

Focused beam over the mirror surface
Although not very useful for producing scanned images of a sample, this regime of operation is of particular interest for applications such as multi-pass electron microscopy where multiple interactions between a stationary beam and a weak phase object inside an electron cavity could lead to more efficient phase contrast imaging [14]. It is crucial for multi-pass electron microscopy that the reflected beam be re-imaged exactly back on the sample in order for the beam to accrue the correct phase. Although in multi-pass electron microscopy the beam incident on the sample must not be focused, the mirror settings, namely having the sample on the image plane of the tetrode mirror, is identical to what we present in this work.

Broad beam over a curved mirror potential
Appropriately curved electron mirrors have been shown to be capable of inducing spherical and chromatic aberrations with sufficiently negative coefficients in order to cancel out the aberrations of the objective lens [10,12]. Here, without attempting to demonstrate quantitative cancellation of spherical aberration, we show that the same imaging technique discussed above could be performed with a curved mirror. To create a curved mirror, we added an aperture electrode to our previous tetrode mirror assembly. In this new five-electrode system, as shown in Fig. 3 Tromp et al. [12].
The reflected image shown in Fig. 3(f) shows qualitative resolution improvements compared to the reflected image in Fig. 3(b). We attribute this improvement in image resolution to the lower spherical and chromatic aber-rations of the curved mirror system compared to those of the flat mirror system. Although, this is not an exercise in aberration correction, it does point towards our ability to change the aberration coefficients of our mirror system by manipulating the curvature of the reflection surface. It is conceivable that our imaging scheme combined with an accurately tuned curved mirror could be used in the future to correct the aberrations of the SEM objective lens without the need for a beam separator.

Reflected image resolution and field of view
The discrepancy in image resolution and contrast between the direct and the reflected images is observed to various degrees in all regimes of operation as apparent in all micrographs presented in this work. The poorer resolution and contrast of the reflected image is to be expected considering the added spherical and chromatic aberrations that the uncorrected mirror system imparts onto the reflected beam. In this section, we use micrographs to quantify the beam spot size after reflection, comment on the contrast of the reflected image, and discuss the useful field of view and magnification of the reflected image.
As a measure of the electron probe size after reflection, we obtained the intensity profile of the edge indicated by the dashed line in Fig. 4(a). nm. Note that this measurement is not of the direct beam whose diameter, devoid of the aberrations of the mirror system, is considerably smaller.
To compare the resolution of the direct and the reflected images, we imaged samples with relatively fine features. We deposited micrometer-sized tin particles on a TEM copper grid with a thin carbon membrane on top.
In certain areas, the carbon membrane ruptured and wrapped around the tin particles. The result, as shown in Fig. 4(c), is suspended tin particles wrapped in thin strands of carbon. The blue and red dashed lines represent the line scans whose profiles are shown in Fig. 4(d). about 152 nm wide, which indicates slight beam broadening after reflection.
More significantly, however, the reflected image shows a one-sided halo to the right side of the sample as evident from the micrograph in Fig. 4(c) and the intensity level of the reflected line profile (red) to the right of each peak in Fig. 4(d). This one-sided halo is resulted from a distorted scanning probe which we speculate could be due to a combination of astigmatism imparted onto the reflected beam by a slightly misaligned mirror system and the residual electric field from the mirror system near the grounded sample.

Conclusion and future work
We demonstrated a scheme for simultaneous imaging of top and bottom surfaces of a sample using multi-electrode electron mirrors in an SEM. We believe that this technique could be of value to the in-situ SEM community where simultaneously observing dynamic changes on both sides of a sample may be of interest. We analyzed the image resolution currently attainable 29 in our setup and identified ways through which this resolution could be improved. Most crucially, by imaging the pivot point of the SEM onto the reflection center, we hope to improve the resolution of the reflected image in areas farther away from the optical axis.
Next, by slightly altering the mirror setup, we demonstrated that the reflected image resolution could be improved when using a five-electrode concave mirror system. We speculate that the smaller aberrations of the curved mirror could be responsible for this improvement. We believe that for the special case of perforated samples, our imaging scheme could allow for "in-line" spherical aberration correction in an SEM without the need for beam separators of any kind. Rigorous analysis as well as fulfillment of the aforementioned design improvements to the experimental setup are necessary for realization of this aberration correction strategy.
In parallel, we have been developing tetrode electron mirrors fabricated using MEMS fabrication techniques. This alternative approach enables the manufacturing of very round lens apertures that eliminate the need for a stigmator to correct for the non-roundness of apertures that is encountered in conventionally machined electrode apertures [26]. Furthermore, the miniaturization of electron mirrors would allow for easier integration in a wider range of commercial SEMs. Finally, due to the smaller electrode aperture and electrode separation of these micromachined mirror systems, it would be possible to maintain similar electric fields as used in this work but with smaller applied voltages. This could open up the possibility of using accelerating lensing as opposed to decelerating lensing which would reduce the spherical aberration for a given focal length.
The potential applications of the imaging scheme reported in this work are not limited only to microscopy; indeed, it is feasible to envision this technique being extended to focused ion-beam milling. Considering that ion beams, much like electron beams, could be reflected and refocused using an electrostatic multi-electrode mirror system, one could incorporate a multielectrode mirror under the sample in an ion-milling system. By programming the scan, the user could achieve ion milling on the top and bottom surfaces of a sample without the need to flip the sample over. Furthermore, since most focused ion beam tools come equipped with an electron column, one could envision using our mirror imaging technique to monitor the bottom surface of the specimen for signs of damage or redeposition while the sample is being milled on the top surface.