A typical focused ion beam (FIB) system using a gallium liquid metal ion source (LMIS) can provide five to seven nanometers of lateral resolution. Such systems are widely used in the characterization and treatment of materials on microscopic to nanoscopic scales. A gallium LMIS typically comprises a pointed needle coated with a layer of gallium. The needle may be maintained at a high temperature while an electric field is applied to extract ions from the source.
FIB systems with gallium LMIS's are used in many applications because of their ability to image, mill, deposit, and analyze with great precision. Milling or micromachining involves the removal of bulk material at or near the surface. Milling can be performed without an etch-assisting gas, in a process called sputtering, or using an etch-assisting gas, in a process referred to as chemically-assisted ion beam etching. U.S. Pat. No. 5,188,705, which is assigned to the assignee of the present invention, describes a chemically-assisted ion beam etching process. In chemically-assisted ion beam etching, an etch-enhancing gas reacts in the presence of the ion beam to combine with the surface material to form volatile compounds. In FIB deposition, a precursor gas, such as an organometallic compound, decomposes in the presence of the ion beam to deposit material onto the target surface.
In ion beam-assisted deposition and etching, a gas is adsorbed onto the specimen surface and reacts in the presence of the ion beam. The rate of material removal or deposition depends on the number of ions striking the target surface, the rate at which gas molecules are adsorbed by the surface, and the number of atoms removed or deposited by each ion.
In all of the processes described above, the function of the gallium ions in the beam is to provide energy, either to displace particles on the work piece in sputtering or to activate a chemical reaction of a molecule adhered to the surface. The gallium itself does not typically participate in the reaction. Gallium is used in the beam because its properties, such as melting point, ionization energy, and mass, make it suitable to form into a narrow beam to interact with commonly used work piece materials.
There are disadvantages to using LMIS'S. With regard to chemically-assisted etching or deposition, because the gallium itself merely provides energy for the reaction and does not otherwise participate, the reaction rate is limited by adsorption rate of the reacting molecules. For example, in FIB deposition, if the ion beam dwells too long at a point, the adsorbed gas molecules are all decomposed and the beam begins to etch, rather than deposit. To mill or deposit, the ion beam is typically scanned repeatedly over a rectangle in a raster pattern. As the beam completes a scan, the beam is typically delayed for a significant amount of time before beginning the next scan to provide time for additional gas molecules to adsorb onto the surface before beginning a new raster. This increases processing time.
Moreover, gallium atoms implant into the work piece and, in many applications, produce undesirable side effects, such as changing the opacity or electrical properties of a work piece. Gallium can also disrupt the crystal structure in the area of bombardment. The type of ion emitted from a LMIS cannot be readily changed, which is a disadvantage because different ion species may be preferred for different processes. To change the ion species, the source must be removed from the vacuum chamber and replaced with a different source, which must then undergo a time consuming preparation procedure. Also, to produce a very narrow beam, the current in a beam from an LMIS must be kept relatively low, which means low etch rates and longer processing times.
Plasma etch systems used in semiconductor manufacturing, unlike beams of gallium atoms, typically use ions in a plasma to chemically react with the work piece. Such systems, however, typically provide a reactive plasma over the entire surface of a wafer and are not used to locally etch or deposit fine features.
Plasma ion sources have been used to form ion beams, but such beams are not typically used to mill or deposit fine features on a work piece because beams from plasma ion sources were difficult to focus into a fine spot while maintaining a useful beam current. Such beams were typically used either to broadly etch a large area, such as to thin samples for viewing on a transmission electron microscope, or to produce a small spot size at low beam current, for example, for secondary ion mass spectroscopy analysis. Moreover, such plasma sources are limited to the specific types of gases and the lifetime of such sources are relatively short with some gases because the plasma would corrode the cathode.
The magnetically enhanced, inductively coupled plasma ion source described in U.S. Pat. Appl. Publ. No. 2005/0183667 for a “Magnetically enhanced, inductively coupled plasma source for a focused ion beam system” can be used to produce a finely focused beam with a relatively large beam current, thereby overcoming many of the problems of a gallium LMIS system. U.S. Pat. Appl. Publ. No. 2005/0183667 describes a system using a single ion species.
There is a need for a system that enables the user to selectively provide gases of different ion species for performing different treatments of a specimen such as milling, etching, deposition and imaging, without requiring replacing the source.