Overview #
FIB-SEMs can be used to deposit a variety of elements onto a sample substrate using beam-induced depositions (BID). During BID, a precursor gas is injected into the FIB-SEM chamber near the sample surface using a gas injection system. The FIB-SEM beams induce the gas molecules to decompose and deposit a solid onto the sample surface. Both the electron beam and ion beams can be used for depositing material, but each beam has its advantages and drawbacks.
Electron Beam Induced Deposition (EBID) #
Using the electron beam to deposit material is known as electron beam induced deposition (EBID). EBID is generally used as the first layer of the deposition because the electron beam energy is low enough that the damage to the top layer of the sample is minimized. However, because of the low energy, the deposition rate of the solids tend to be low. The result is that EBID tends to be used in applications where 10’s of nm of solid depth is needed. For larger volumes of deposit, IBID is used on top of the EBID.
Ion Beam Induced Deposition (IBID) #
Ion beam induced deposition (IDIB) is used when large volumes of solid needs to be deposited. Depending on the ion source and the deposit area, IBID depths can accumulate to several microns in just a few minutes. Because the ion beam is highly energetic, care must be taken to match the beam current to the amount of precursor gas at the surface or the sample will be etched rather than solid deposited (Figure 1).
During the initial stages of the deposition, imaging of the deposit with the electron beam in a FIB-SEM can be used to monitor the growth of the deposition. If etching is observed, lowering of the beam current can be used to begin deposition. The optimal beam current density to deposit a material will be dependent on the exact instrument setup, and the instrument vendor should supply target current densities.
Most depositions typically occur in the 5 to 10 pA μm-2 range. For example, if a deposit of 5 x 25 μm Pt rectangle is desired, the beam current for the deposition should be between 0.6 and 1.2 nA :
5 pA μm-2 * (5*25 μm) = 5*125= 625 pA = 0.6 nA
10 pA μm-2 * (5*25 μm) = 10*125= 1250 pA = 1.2 nA
Precursor Gases #
A wide variety of gaseous precursors can be used to deposit materials for BID. The chemicals tend to be organo-metallic compounds of elements to increase the volatility of the metals. In Table 1, an small list of common precursors are listed in Table 1. A more complete list of deposition precursors can be found in the Practical Electron Microscopy and Database book.
| Deposition Compound | Deposition Compound |
|---|---|
| C | Phenanthrene: C14H10 |
| Al | (CH3)3NAlH3 |
| SiO2 | Si(OCH3)4 + O2 |
| W | W(CO)6 |
| Pt | (CH3C5H4)Pt(CH3)3 |
| Au | (CF3C(O)CH2C(O)CF3)Au(CH3)2 |
The deposits made during BID are composed of a mixture of the different elements present in the precursor gases, along with the possibility of the beam to incorporate. For example, the deposit formed by depositing from (CH3C5H4)Pt(CH3)3, a common Pt source, contains both Pt as well as C. The ratio of Pt to C will depend on the beam type (electron vs. ion), the beam current, and the sample temperature.
Ion-Gas Interactions #
The process of BID deposition occurs when the precursor gas is imparted energy from the incident beam. The precursor gas then decomposes into a non-volatile species and deposits onto the surface. Depending on the precursor chemistry, some of the ligands from the precursor may be still volatile and be removed from system by the instrument vacuum system. The non-volatile species deposited can be a mixture of the metal and the ligands present from the precursor, resulting in a deposit that is a mixture of elements.
The energy of the incident beam has a large effect on the precursor decomposition process. Enough energy needs to be imparted to the precursor molecule from the incident ion/electron to break the metal-ligand bond and decompose the precursor. As a result, higher deposition rates are observed with heavier ion species (Ga is faster than He is faster than an electron) and with increasing beam accelerating currents. The increasing energy also will effect the purity of the deposit, but will also depend heavily on the incident beam species.
Cryo-BID #
The substrate temperature greatly effects the rate of BID. Lower temperatures will cause the precursor gas to accumulate at the surface, resulting in a larger flux of gas to the surface. As a result, much higher deposition currents can be used to deposit material. For example, it has been observed that Pt deposition at -150°C is four orders of magnitude (10,000X) faster than at 25°C (Grigorescu and Hagen), while the purity of the deposit is similar at both temperatures.
