Patterning issues and strategies to deal with them.
Patterning issues
Causes and solutions
Mill profile: rounding of the edges and inclined sidewalls
Caused by the combination of Gaussian beam profile and by the fact that the sputter yield is larger at small angle of incidence (i.e., on sidewalls), and in some cases by re-deposition Edges that are particularly important to be sharp should always be finished last in order and with a small beam current If the beam profile and the scan style cannot be further improved, an additional, sacrificial thin film of material might be coated on the sample, which would take up much of the rounding and that would be removed after IBL patterning In some applications, it may also be useful to consider tilting the sample a couple of degrees from normal incidence in order to produce edges that are vertical to the sample surface
Mill profile: uneven surfaces and poor pattern definition due to differential milling
Caused by the fact that milling rates depend on crystal orientations present in the sample (strongly channeling orientations sputter less) “Thin films of polycrystalline material on an amorphous or single-crystalline substrate are best milled with a small number of passes and a long pixel dwell time The obtainable definition of the pattern will in most cases outweigh the FIB milling artifacts in the substrate due to long dwell times” [11]
Surface roughening/ripple formation
Surface roughening and ripple formation are reported on single crystal targets as well. It is attributed to the competition between the changing sputter yield with angle of incidence (drives roughening at certain patterning parameters set) and surface diffusion (that might help smoothing). See [6] for more on this
Swelling
Swelling is the geometrical consequence of ion incorporation into the sample material and of the destruction of crystalline order. When the ion beam incidence is rather normal to the sample surface, swelling is often observed in early stages of the milling process at low ion doses. Swelling also contributes to the general mill profile by raising up material near the milled edges. See the entry “knock-on damage and incorporation of ions” (below) for prevention and treatment
Mill profile: unsatisfactory depth and/or shape due to redeposition of sputtered material inside pits
Example mill profiles affected by re-deposition, as well as “clean” results achieved by increased number of repetition, are shown in Figure 2. It is important to note that as the aspect ratio (depth to width ratio) of a milled pattern is increased, it becomes less and less frequent that the sputtered material is projected out of the hole or trench during milling. At around 10 : 1 aspect ratio, the milling effectively stops regardless of scan style or repetition number It is possible to increase aspect ratios (as well as to speed up the milling process) by gas-assisted etching. When a chemically reactive gas is introduced at the site of patterning, the milling can be enhanced due to gas-phase reaction products that escape from the high aspect ratio patterns. Using reactive ion etching will change the mill profile in other ways too, dependent on actual chemistry, pressure/flow rate, ion energy, and so forth
Re-deposition
In general, re-deposition of sputtered material is a difficult issue with complicated, large volume, or 3D designs, especially if the milled pattern is physically enclosed in its location, or if closely surrounded by other lithography features In most cases it is feasible to finish milling with the outline of the finest (or most sensitive) structure, polishing up with small beam currents. Moreover, the use of an etching gas increases the volatility of the removed material and can help to minimize re-deposition
Charging and positional drift
If the sample is electrically insulating or involves smaller size electrically isolated islands, charges will not be able to dissipate at the site of patterning. Charging can cause drifts while accumulating; melting and explosions can occur during discharge. Charging may be prevented by applying one or more of the following: coating the surface with a conductive thin film, assisting opposite charges with an electron gun, or assisting a nonreactive gas locally (that could be water vapor dependent on the target material). Automated drift correction, in which a fiducial marker (reference point) is visited periodically for detecting and correcting shift in positioning, should take care of minimal charging issues and instrument positional drift in general. Also see [135, 164, 165]
Knock-on damage and the incorporation of ions into the sample
The unavoidable but limitable and sometimes curable side effect of ion beam processing is the undesired change of atomic order in the interaction volume. In particular at higher beam energies, the collision cascade can reach deeper and wider (see Figure 1 for a reference in silicon) There are four aspects of dealing with knock-on-damage. () The damage can be directly reduced by reducing the ion beam energy. This is exactly why low kV polishing is commonly employed as a last step in TEM or APT sample preparation () The damage may be recovered by treating the sample after IBL. Annealing is often reported as a successful means to heal crystal defects or recrystallize, and sometimes it is possible to expel the incorporated ion specie content () In general, the damage can be minimized by scanning only outer parts of the sample and the reference or alignment marks and using “blind” navigation and pattern placement. Then just the processing dose itself is applied to the crucial or sensitive parts of the sample () Part of the processing dose damage is also avoidable. Unintended ion exposure from beam tails and from ion blanking lines may be blocked by coating a protective thin film on the surface (for hard masking). The protective coat may be removed later easily, if its material is chosen suitably and is compatible with the sample material Note that crystalline samples might go through amorphization, alloy phase formation, and/or ion beam induced grain growth. See also [3]. Also note that these changes can be beneficial and be taken advantage of in some nanofabrication processes. Examples are listed and discussed in Section 5
Shadowing on surfaces and curtaining effect on cross-section cuts
Uneven topography in the line of sight of ions causes shadowing and curtaining. Even if the original surface is flat, uneven topography can develop if the sample exhibits differential milling, and/or if it is composed of various materials of different sputter yield. See also [3]
Deterioration of fiducial markers
Caused by unintended milling of the fiducial markers during observational scans. Fiducial markers (also referred to as alignment marks) mark specific locations on the sample to be used for drift correction, pattern overlay, and write-field alignment. Fiducial markers may be produced by various methods (dependent on the place of IBL in the work flow). Besides depositing hard-to-sputter, small grain, or amorphous and conductive materials to fabricate fine resolution alignment patterns, topographical markers should be considered that can be deep etched into the sample, sometimes even by the IBL instrument itself. Markers may be scanned over hundreds of times and sometimes with relatively large ion beam currents (whatever is used for the IBL processing step), so they have to be durable enough to serve as needed. In addition, the concept of a “dynamic mark reference” can be employed, in which the reference image is repeatedly redefined in order to account for changes in shape and size (i.e., after a few mark scans a freshly acquired scan image is saved as the new reference and used for the following alignment procedures)
