When using an ion-beam to modify a sample, various ion-solid interactions can cause undesired structures known as milling artifacts. This article will discuss the most common milling artifacts and methods to prevent them.
Redeposition #
During the FIB milling process, the sputtered material can redeposit onto other areas of the sample, resulting in undesirable topographies and sidewall sloping. Anticipating the most likely redeposition area based on the milling direction is possible. Typically, redeposited material is found at the back of the milled pattern (left side if milling from left to right, bottom if milling bottom to top). To mitigate the severity of redeposition, milling trapezoids instead of rectangles is recommended. This enlarges the redeposition area, minimizing the impact.
Channeling #
When milling into a sample using an ion-beam, particularly with crystalline samples, the orientation of the atoms in the sample relative to the ion-beam can influence the milling rate. Areas of the sample with higher atomic density tend to mill faster compared to areas with lower relative density. We refer to this variation in milling rates as channeling.
Figure 1 showcases a milled rectangle on a multi-grained copper surface, allowing easy observation of the copper’s grain structure with superior channeling contrast in ion images. The brighter grain in the copper exhibits higher atomic density in the ion-beam direction, generating more secondary electrons during ion-beam imaging. The sample’s milling process results in faster rates in the bright grain, while the darker grains show well-defined sidewalls due to lower sputter yield. Excessive sidewall slopes occur in the faster milling bright grain due to increased redeposition caused by faster sputtering. Patterning across grain boundaries can lead to uneven sputtering and reduced structure quality. Enhance structure quality by employing slow milling grains.
Curtaining #
The incident angle of the ion beam affects the sputtering yield, which, in turn, is influenced by topographic features. The schematic on the left demonstrates this relationship. Curtaining, an artifact that produces vertical lines on the cross-section surface (see SEM image), can occur due to increased surface roughness. To mitigate this artifact, you can apply a smooth deposition layer or a single crystal mask on top of your sample. Additionally, reducing ion beam currents during cross-section polishing and utilizing a rocking stage can help minimize curtaining. SEM images showcase the presence of curtaining and how it can be reduced using these approaches.
Beam tails #
The quality of the patterns is highly dependent on the beam profile. The ion beam has a Gaussian profile. The sides of the ion beam are called beam tails. Only a few ions are within these tails, however, they can also remove sample material and create undesired structural changes to your sample, such as slopes. The better the beam profile, the smaller the created slopes. The ion beam has a natural Gaussian shape. This means that the majority of ions hit the sample within the designated beam spot. However, a non negligible part of the ions are present in what is called the ‘beam tail’. These ions of course interact with the sample as well and can do some damage. They cause sidewall sloping and over cutting. You can see in the beam tails in the spot burn pattern (bright rings around the holes). Beam tails cause patterning outside your pattern area. The larger the beam tails, the worse the structure quality. The beam profile of the HIM for He and Ne is superior to that of a Ga FIB and therefore allows you to structure higher quality nanostructures. An example of this is shown on the slide where a 200nm diameter ring with a 30nm width was structured. The large beam tails for Ga (left) cause slide wall sloping and also start to round off the structure in the middle. The structure quality when cut with He is much better which can be attributed to smaller beam tails (as well as slower sputtering and therefore reduced redeposition).
Material Interfaces #
Different materials have different sputter yields. The left image shows the effect of cross-sectioning across a material interface. Different depths are achieved when cutting across different materials. TEM lamellas can break at the interface, as shown in the right image. Possible solution cross-section: => cut cross-section deeper until the desired depth for the slow milling material is reached Possible solution Lamella: => Attach lamella 90 rotated
Heat damage #
The incident ion’s energy is predominantly deposited in phonons and converted to heat (electronic energy losses (inelastic) also contribute to this). The temperature difference which is caused by the ion-solid interactions depends on the sample’s thermal conductivity, the sample geometry, the FIB system setup and sample-heat reservoir contacts. For Si, the temperature increases less than 2°C which has often lead to heating effects being neglected. Biological samples or polymers are more prone to heating and very high temperatures can be reached in such samples. Heat damage is visible as unnaturally smooth surfaces which can also present with voids, like in the SEM image above.
Amorphization #
The ion beam – sample interactions lead to sample atom displacements. With sufficiently high doses (number of ions per area) a lot of sample atoms are displaced in the first few nanometres of the sample and the sample crystallinity can be lost, resulting in amorphous layers (as shown in the HRTEM images above). The TEM image shows an amorphous layer which formed in silicon when using 30keV Ga ions (left), after a 5keV Ga (middle), 2keV Ga (right). The amorphous layer is indicated by the blue arrow. The size of the amorphous layer corresponds to the interaction volume width. The corresponding SRIM simulations above show the simulated interaction volumes for each image. Sample amorphization depends on the unit cell size, chemical ordering complexity, the width of the intermetallic phase field. In general samples with a larger unit cell, a complex unit cell, compounds and intermetallics are more prone to amorphization. The thickness of the amorphous layer directly corresponds to the ion energy: lower energies reduce the amorphous layer thickness. It also depends on the ion species and the resulting interaction volume: He has a larger amorphous layer than Ne than Ga than Xe, because lighter ions travel further in solids before stopping. The reader is also referred to the older literature on amorphization in ion beam thinning, and there is literature on this topic dating back several decades. It should also be noted that damage other than amorphization can also occur, such as collection of ions from the beam into bubbles, segregation features, alloying with the material, or inducing crystal structure change.
References on amorphization and damage #
- Reduced damage with Xe beam liftout
- Lattice distortion due to ion implantation
- Phase transformation in cobalt due to Ga implantation:
- Phase transformation in FeAl due to Ne implanatation:
- Ga-induced phase transformations in steels:
- Ga- and Xe-induced phase transformations in ZrO2
- Damage in metallic and semiconducting materials due to Ga
- Texture development and grain growth in fine-grained materials
Image/Scan artifacts #
A single scan with the ion beam can severely damage the sample and introduce artefacts. How excessive the sample damage is depends on the sample, the used ion dose and ion species. Be careful with the ion beam (especially with Ga and Xe). The image recorded with Helium does not show as much damage as the one recorded with Ga. Both images recorded using the same dose (1.1E16 ions/cm2). You have seen on previous slides that Ga ions induce more sample sputtering and sub surface sample damage (dislocations, vacancies etc).
Hydrocarbons: Lighter ion species such as He,Ne #
Hydrocarbons are molecules which consist of carbon and hydrogen. They can have various forms and are always present in the air. They easily adsorb onto samples and are then transferred into the HIM. The ion beam cracks the bond between the carbon atoms and the hydrogen atoms in the electronic interactions. The bond cracking is displayed as purple flashes in the schematic on the right hand side. Once the bonds are cracked, the hydrogen gets pumped away by the vacuum system. The carbon rests deposit on the sample. This can lead to deposition rather than milling when attempting to produce small nanostructures. This artifact can predominantly be found when milling small structures with He. He has a low sputtering yield and predominantly interacts with the sample atom electrons leading to broken bonds in hydrocarbons. The non-volatile carbon parts deposit on the sample faster than the He can mill the sample away. If you encounter this problem you need to plasma clean your sample as well as ensure that your system chamber is as clean as possible (a lot of FIBs have an inbuilt plasma cleaner for the chamber to remove hydrocarbons).
Ion implantation #
The beam ions get stuck in the sample after losing all their energy in the ion-sample atom interactions. This is an undesired artefact. In semiconductors, Ga ions are a dopant which strongly alter the physical properties of the sample (e.g. conductivity). Patterning with He and Ne or Xe has the advantage that these ion species do not cause Ga-poisoning of the sample or change their physical properties. However, they still implant. Parameters affecting implantation: ion energy, angle, ion species, target material
Interface mixing #
Interface mixing occurs when atoms from 1 sample layer are transported into the next sample layer. Ions transfer enough energy to recoil atoms in layer 1 which can move into layer 2, leading to layer contamination. Atoms can be recoiled towards the surface. This is caused not by the ions’ created cascade but by the cascade created recoils. Recoil cascades start forward but become isotropic (all directions are equal). The TEM image shows the intermixing of 2 Pt layers (electron beam deposited Pt and ion beam deposited Pt). The damage layer is ~50nm thick. Intermixing occurs a lot for heavier ions such as Xe.
Phase transformations #
FIB, especially using Ga ions, can induce phase transformations in the sample. An example is shown in the image where part of a copper sample was irradiated with 30keV Ga ions. The TEM analysis of that area shows that the sample has phase transformed from Cu to Cu3Ga (as a result of the Ga implantation). Crystallographic reoriention towards a preferred orientation have recently been reported in literature.
