This invention relates to micro-machining copper using a charged particle beam in the presence of an etching assisting agent.
Aluminum, tungsten, and copper have been used for metal interconnections in integrated circuits. Copper is particularly desirable, especially in high performance integrated circuits, because of its low resistivity. In a typical operation, the copper is deposited into spaces micro-machined, using a global lithographic process, into the surface of a dielectric material such as silicon dioxide, to yield a series of metal interconnections in a predetermined pattern.
Charged particle beam micro-machining, assisted by a specific chemistry process, has been a useful process for local micro-machining metal interconnections during integrated circuit design and troubleshooting. Focused ion beam (FIB) micro-machining has been particularly useful in this regard, especially in debugging, editing, validation, metrology, and process control. In charged particle beam micro-machining, a metal trace is exposed to a beam of, e.g., gallium ions, often in the presence of a specific chemistry to assist removal of the metallization. Charged particle beam micro-machining has been used successfully to micro-machine aluminum and tungsten traces, resulting in fast and clean cutting.
Problems have arisen, however, in the case of copper. Certain chemistries, such as those based on iodine are generally inapplicable to copper metallizations. Iodine-based chemistries can spontaneously etch copper and corrode exposed copper within hundreds of microns from the initial beam exposure point. This corrosion can present serious reliability issues such as high resistivity, electrical leakage, and ultimately, conductor failure.
Copper traces consist of micro grains with various crystallographic orientations, causing the micro grains to etch at different rates. The presence of these micro grains leads to strongly pronounced channeling in the copper at some orientations. The net result is that the copper etches unevenly, thereby limiting the utility of charged particle beam processes in micro-machining copper interconnections. Furthermore, an underlying dielectric layer can experience some damage due to the uneven etching.
What is desired is a process that enhances the use of charged particle beam processes so that etching can be performed with minimal re-deposition and provide clean electrical isolation. It would also be desirable to provide a process that gives the capability of precisely milling copper, to ensure high isolation resistance so as to provide the electric isolation necessary for circuit modifications.
The invention provides a process for micro-machining semiconductor materials that advantageously improves the etching and separation (e.g. cutting) of copper interconnections. Deeply buried traces can be cut in a substrate using the inventive process.
In one aspect of the invention, the process includes micro-machining a substrate in the presence of an etch assisting agent using a charged particle beam and thereby, forming a disconnection or separation in the copper. The process includes: (a) providing a substrate that includes a dielectric layer and copper overlying the dielectric layer; and (b) etching the copper with a charged particle beam in the presence of an etch assisting agent. The etch assisting agent can include ammonia, acetic acid, thiolacetic acid, or combinations thereof.
The invention provides a process for etching copper using a compatible chemistry, where the chemistry assists in achieving a relatively high selectivity of copper with respect to the underlying dielectric layer, resulting in cleaner, more even milling and cuts. By increasing the selectivity of copper, surface roughness development is decreased under ion bombardment. Reductions are also achieved in the re-deposition of the etched material and the electrical conductivity of any material that may be re-deposited.
The following terms are intended to have the following meanings:
xe2x80x9cSelectivityxe2x80x9d is the ratio between the removal rates of copper (Cu) and the dielectric material (measured under the same conditions); and
xe2x80x9cRemoval ratexe2x80x9d and xe2x80x9cetch ratexe2x80x9d is the average thickness change per unit time, and is provided in e.g., microns/per sec.