Gas Injection System

Table of Contents

Overview #

Gas injection systems (GIS), introduce reactive gases to a sample surface with a needle and the gas is subsequently ‘cracked’ by either the electron or ion beam forming volatile components which are pumped away by the pumping system, as well as non-volatile components which interact with a sample (Figure 1). GIS are used primarily for material deposition, but also enhanced and preferential etching. Today 26 different chemical precursor gases are used in GIS that are available for a wide range of applications.

Figure 1. Schematic of ion beam induced deposition
of Pt with a gas injection system.

Uses #

Material Deposition #

GIS also allow the deposition of a broad range of materials including: platinum, tungsten, carbon, and other solids, onto a sample using beam induced deposition. During deposition, a gaseous precursor (e.g. (CH3)3Pt(CpCH3) for the platinum rich deposition) is broadly released onto the sample from a GIS needle. The precursor molecules are then ‘cracked’ by the a beam into volatile products and non-volatile products. The non-volatile products (here Pt-rich solid) adsorb on the sample surface while the volatile products are removed by the chamber vacuum system. The deposited materials generally contain impurities from the precursor ligands (C in the above example). A significant amount of work has been invested in improving the purity of the deposit in recent years, including cryo-IBID and deposition under low vacuum conditions using the electron beam.

Figure 2. A ion-beam induced deposition Pt protective cap on a sample.

Material deposition can be used for micro and nanofabrication and microelectronic circuit editing, but is most often used to deposit layers on the surface of a material to protect the underlying sample from ion beam damage during milling (Figure 2). Protection layers can also help reduce curtaining artifacts.

Enhanced and Preferential Etching #

Gas assisted etching can be used to enhance sputtering rates of materials or to preferentially mill one material over another. Both types of etching applications are commonly used in the semiconductor industry. For example, the Epsom salt precursor increases the milling rate of carbon and resist materials and reduces the milling rate of some metals, allowing one to preferentially mill the resist and not the metals in semiconducting devices.

Operational Safety Considerations #

Chemical Hazards #

Each precursor molecule from a vendor should have a safety data sheet (SDS) and it is important to familiarize oneself with the chemical that is inside the GIS and understand the implications of working with the chemical. Many of the precursors are very hazardous. It is recommended to contact the vendor and ask for the updated SDS for the precursors. Another good source for the SDS is Sigma Aldrich.

Working Distances #

GIS needles are inserted as close as 50 to 200 µm from the sample surface in most FIB-SEM systems. The distance between the inserted GIS and the sample is therefore approximately the same or less as the width of a human hair! The the working distance of the instrument (or eucentric height) is the safe working distance at which the GIS can be inserted without collision for flat samples. If the stage is too close to the pole piece, the GIS needle will collide with the stage and cause GIS and sample damage since there is no way to stop GIS insertion.

Stage Tilt #

For many systems, the GIS needle can be inserted both with a tilted stage as well as with an non-tilted stage. However, one should check carefully in the operating procedures of each instruments. There are FIB-SEM systems which cannot operate the GIS when the stage is not tilted and an attempt to do so would result in damage to the instrument and sample. Check with the vendor for each instrument for safe operations procedures.

Chamber Geometry #

It is good practice to know the chamber port and direction that a GIS needle is inserted into the chamber and what the distance to the stage is for the commonly used stage tilts. Understanding the geometry is critical knowledge, especially when working with uneven samples. A good way to understand the geometry is to take SEM and FIB images with the GIS inserted and to ask the service engineer about the stage position at which the GIS would touch the stage (Figure 3).

Figure 3. Secondary electron micrographs of a GIS needle as viewed from
the electron beam (left) and from the ion beam (right).

Stage Movements #

It is good practice to insert the GIS as a last step before patterning and retracting the GIS immediately after the patterning has finished. As a result, accidental stage movements which can cause collision with the GIS are reduced. Samples are seldom perfectly flat or mounted flat and even small movements can bring sample features into GIS collision range. It is recommended to reset eucentric height every time the stage was moved. For some processes, such as TEM lamella preparation, this practice may not be applicable and great care must be taken whenever the GIS is inserted.

Operation Conditions #

Temperature #

The GIS typically must be warmed up to working temperature before it can be used. Every precursor material has a different temperature setpoint. These are conventionally set by the instrument service engineer.

Deposition Beam Current #

To successfully deposit the precursor material, approximately the same amount of ions and precursor molecules need to be over the deposition area at the same time. To achieve this, the ion beam current needs to be calculated/selected in dependence of the deposited structure size (see Area 2 in Figure 4). Calculating the required ion beam current for the structure size ensures that there is the right amount of electrons or ions available to ‘crack’ all the deposition precursor molecules. The ideal current produces a current density (current per surface area)  between 6 and 10  picoamps per square micron (pA / μm2) for ion beam induced depositions.

Figure 4. Schematic of the beam induced deposition rate as a function of beam current density (bottom) with micrographs of ideal depositions (top left) with adequate current density and sample milling (top right) with too high of beam current density.

Using a current density that is too low results in underutilization of the gas precursors, which results in slower deposition rates, longer deposition times, and leaves precursor molecules within the chamber causing further contamination. This corresponds to Area 1 in Figure 4. If the current density is too high all gas precursor are consumed by the beam and the remaining beam energy sputters the sample surface, milling the pattern instead of causing deposition. This corresponds to Area 3 in Figure 4.

References #

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