We investigate the ability of a focused helium ion beam to selectively modify and mill materials. The sub nanometer probe size of the helium ion microscope used provides lateral control not previously available for helium ion irradiation experiments. At high incidence angles the helium ions were found to remove surface material from a silicon lamella leaving the subsurface structure intact for further analysis. Surface roughness and contaminants were both reduced by the irradiation process. Fabrication is also realized with a high level of patterning acuity. Implantation of helium beneath the surface of the sample is visualized in cross section allowing direct observation of the extended effects of high dose irradiation. The effect of the irradiation on the crystal structure of the material is presented. Applications of the sample modification process are presented and further prospects discussed.

Ion beams are widely used to modify the physical and chemical properties of the surface of materials with a high degree of control. Ion beam irradiation can be used to modify and control a material’s optical [1], electrical [2], magnetic [3] and mechanical [4] properties. The gallium focused ion beam (FIB) microscope has been commercially available for twenty years. FIB microscopes have proven themselves as versatile tools with applications in a range of fields including biology [5], geology [6], materials science [7–8] and the semiconductor industry [9]. While the FIB has been adopted for many uses it is not without its limitations. The FIB uses gallium ions, a metallic element which is often considered a contaminant. The large momentum of the gallium ions in the FIB can have a very destructive effect on materials, greatly altering their crystal structure. The resolution of the FIB is limited by the energy spread of the gallium ions generated from the liquid metal ion source (LMIS). The sputter yield is also too large for acute patterning control over very short lateral distances. The recently developed Carl Zeiss Orion Plus helium ion microscope (HIM) is a new type of focused ion beam microscope. The HIM uses helium ions instead of gallium ions. Helium ions have a lower mass and therefore are less destructive than gallium ions. Helium ions are effectively non-contaminating. The source is a gas field ion source which does not suffer the energy spread and subsequent chromatic aberration which limits the resolution of the FIB [10]. Our HIM is capable of sub-nm resolution imaging with its <0.75 nm probe size. This makes it ideally suited to both high resolution imaging and also modification of materials with a higher level of control and precision than can be offered by other ion beam tools. The HIM has the unique ability to directly mill arbitrary patterns with sub 10 nm feature sizes. To date most of this work has been done on graphene [11–13]. The HIM can also directly write sub 10 nm features via precursor gas decomposition [14–15]. The depth of implantation of helium ions in a material can be varied by adjusting the beam energy [16]. Subsurface voids can be produced in such a fashion. Further exposure and subsequent growth of the void can be used to separate a thin film of material from a substrate [17], or to modify the optical properties of the surface [18]. Some other applications to date include imaging of chemical variations at high resolution [19], quantitative dopant contrast mapping [20] and imaging of uncoated biological materials [21]. In this work we further investigate the ability of the HIM to modify a material’s surface and structure using advanced transmission electron microscopy (TEM) techniques such as energy filtered TEM (EFTEM) and electron energy loss spectroscopy (EELS). We also present the limitations of this surface modification technique.

We have demonstrated that a focused helium ion beam can modify a surface’s physical properties, such as crystallinity, roughness and thickness, in a controlled manner. 35 keV helium ions were used to produce a surface which was smoother than could be achieved by 30 keV FIB. Low energy FIB polishing can also improve the lamella quality; however the HIM polishing step has many benefits over the relatively broad beam of low energy FIB due to the its small probe size and the ion species used. The helium ion beam is non-contaminating and can even be used to selectively remove surface contaminants, such as the gallium contamination removal demonstrated here. Thickness dimensions can also be reduced to just a few tens of nanometers. This is critical for techniques in aberration corrected TEM such as atomically resolved EELS [30]. The higher imaging resolution of the HIM than the conventional SEM beam used in dual-beam FIB systems means that smaller features of interest can be located during sample preparation, prior to TEM. Clearly one application of this technique is as a highly useful step in the production of high quality TEM lamellae of silicon based devices, as well as a broader range of materials as demonstrated by our dramatic TiO₂ sample quality improvement. Beyond TEM sample modification, it has been shown that a finely focused beam of helium ions can sputter material from a sample with a high level of control, allowing sub 10 nm features to be patterned. This high dose irradiation can also be used to modify the structure of a material, as demonstrated by our EELS and HRTEM results. The new models of the HIM feature variable acceleration voltage operation, our microscope is in the process of receiving this upgrade and further work will need to be done to assess the effect of reduced beam energies on sample modification. Helium ion irradiation is a widely studied field in nuclear physics. Helium ion irradiation has been used to modify mechanical [31], optical [25] and magnetic [32] properties of surfaces. The highly focused probe of the helium ion microscope provides a greater level of spatial control than previously available for such experiments.

An intrinsic silicon substrate was used as a base material for our experiments. The silicon was mounted on a lift-out sample holder and inserted into a Carl Zeiss Auriga CrossBeam FIB-SEM. A 12 µm long, 0.6 µm wide and 6 µm deep section of silicon was removed from the sample and transferred to an Omniprobe TEM lift-out grid by the in situ lift-out technique [33]. 30keV gallium ions were used to thin the silicon lamella to approximately 200 nm. The final thinning, to ≈100 nm, was done with a 20 pA beam of 30 keV gallium ions until the sample became almost transparent to a 5 keV electron beam, this is an indication that the sample is of sufficient thinness for TEM analysis. This is a common technique for TEM sample preparation. This process was then repeated to produce a second sample. Sample 2 is composed of TiO₂ and was subjected to the same process as sample 1. Sample 3 is a silicon sample; it was treated with an extra final step. The sample face was tilted 2° into the beam and scanned with reduced energy gallium beam of 5 keV for one minute on each side, the beam current was 20 pA. A short dwell time and a large number of scan repeats were used for this step. This step is known to produce high quality TEM lamellae with a FIB induced amorphization and implant layer as thin as ≈2 nm, roughly ten times less than the damage layer produced by 30 keV gallium ions [23]. Sample 1 was then inserted into our HIM. The HIM beam energy was fixed at approximately 35 keV for all experiments. The lamella was loaded with the normal of the sidewall surface perpendicular to the ion beam, the sample was then tilted 1° into the beam. This geometry has been shown to produce surfaces with minimal damage and implantation layer thickness and reduced surface roughness in the FIB. A focused beam of helium ions was then scanned over a 500 nm wide region of the lamella sidewall. This step was conducted a number of times in adjacent regions in order to optimize the dose. The area used in our analysis was exposed to a dose of 3.4 × 10¹⁶ ions/cm². The beam current used was 1.2 pA. The beam was rastered in a single scan over the area with a pixel spacing of 1 nm and a dwell time at each point of 1.3 × 10⁻³ s. The sample was rotated through 180° and the process was repeated on the opposite sidewall. The scanning time per sidewall was 68 s. This section of the sample was observed to have a reduced thickness after modification with helium ions. To investigate controlled modification for different materials sample 2 was treated with the same process. Sample 3 was then loaded into the HIM in the same upright manner. This sample was tilted 15° to the beam and a 300 × 200 nm area was exposed to a dose of 6.2 × 10¹⁸ ions/cm². The beam current used was 3.4 pA. The beam was rastered in a single scan over the area with a pixel spacing of 0.7 nm and a dwell time at each point of 5 × 10⁻³ s. The total scanning time was 612 s. This process produced a hole straight through the lamella. The purpose of this exposure geometry was to produce a wedge shape of silicon within the lamella so that we could observe the minimum thickness dimensions which can be fabricated by HIM. It also allows us to analyze the subsurface modification effects due to helium ion implantation. All helium ion patterning was performed with the integrated pattern generator on the tool. For detailed analysis of the effects of our HIM modification these samples were analyzed in an FEI Titan 80-300 (S)TEM operating at 300 kV. High angle annular dark field (HAADF) images were recorded. HAADF images contain contrast due to sample thickness and composition. Thickness maps of samples 1 and 3 were recorded, these maps use energy filtering of electrons to provide quantitative thickness information. Energy filtered TEM (EFTEM) mapping of the gallium concentration and distribution in sample 1 was recorded. EELS spectra were recorded from sample 3. High resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns were also recorded.