The use of microelectronic devices, such as integrated circuits, flat panel displays and micro electromechanical systems, has burgeoned in new business and consumer electronic equipment, such as personal computers, cellular phones, electronic calendars, personal digital assistants, and medical electronics. Such devices have also become an integral part of more established consumer products such as televisions, stereo components and automobiles.
These devices in turn contain one or more very high quality semiconductor chips made from silicon wafers containing many layers of circuit patterns. Typically nearly 350 processing steps are required to convert a bare silicon wafer surface to a semiconductor chip of sufficient complexity and quality to be used, for example, in high performance logic devices found in today's personal computers. The most common processing steps of semiconductor chip manufacture are wafer-cleaning steps, accounting for over 10 % of the total processing steps. These cleaning steps are normally one of two types: oxidative and etch. During oxidative cleaning steps, oxidative compositions are used to oxidize the silicon or polysilicon surface, typically by contacting the wafer with aqueous peroxide or ozone solution. During etch cleaning steps, etching compositions are used to remove native and deposited silicon oxide films and organic contaminants from the silicon or polysilicon surface before gate oxidation or epitaxial deposition, typically by contacting the wafer with aqueous acid. See, for example, L. A. Zazzera and J. F. Moulder, J. Electrochem. Soc., 136, No. 2, 484 (1989). The ultimate performance of the resulting semiconductor chip will depend greatly on how well each cleaning step has been conducted.
Microelectromechanical systems (MEMS) (also called micromachines or micromechanical devices) are small mechanical devices that can be made using traditional integrated circuit manufacturing techniques. Typical devices include motors, gears, accelerometers, pressure sensors, actuators, mirrors, personal information carriers, biochips, micropumps and valves, flow sensor and implantable medical devices and systems. The manufacture of MEMS results in a chip, or die, which contains the moving pieces of the device made from silicon or polycrystalline silicon (polysilicon) encased in silicon oxide. The die can also contain the circuitry necessary to run the device. One of the final steps in the manufacture of MEMS is commonly referred to as release-etch and consists of an aqueous etch utilizing hydrofluoric acid (HF) to remove the silicon oxide to free, or release, the silicon or polysilicon pieces and allow them to move.
For etch cleaning steps, the composition of choice has been dilute aqueous hydrofluoric acid (HF) and, to a lesser extent, hydrochloric acid (HCl). Currently, many semiconductor fabricators employ an "HF-last" etch cleaning process consisting of an etching step using dilute aqueous HF to etch oxides.
Another important cleaning process in semiconductor chip manufacture is the removal of residues left behind from plasma ashing or etching of dielectric, photoresist or metals. The removal of these "post-etch residues" is challenging because of their multicomponent nature (i.e., the residues are typically comprised of both organic and inorganic compounds) and because the residues are adjacent to sensitive device features that must not be damaged during residue removal. Etch cleaning processes directed at removing "post-etch residues" will often utilize an aqueous HF composition in a first step, followed by a multi-step process to remove inorganic components of the residue. For instance, ethylene glycol-HF-NH.sub.4 F aqueous solutions are widely used for the removal of "post-etch residues" from metal lines, and dilute aqueous HF is often used to remove cap and side wall veil residues after shallow trench isolation etching. See, for example, S. Y. M. Chooi et al., Electrochem. Soc., Proceedings, "Sixth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing," 99-35 (1999).
However, etch cleaning of silicon surfaces with aqueous HF compositions has presented many problems to the semiconductor chip manufacturer. For example, contact with aqueous HF compositions renders the silicon surface hydrophobic and thus very susceptible to contamination by particles such as silicon oxides and other inorganic and organic materials. To remove these particles, the etched wafer is typically rinsed with deionized water, ethyl alcohol or isopropyl alcohol and is dried prior to subsequent processing. Unfortunately, the rinse does not always effectively remove these residual particles from the wafer, as the low energy silicon wafer surface is not easily wet by aqueous or alcoholic rinsing compositions which inherently have high surface tensions. In addition, rinsing with DI water gives rise to slow drying time, while rinsing with alcohol introduces a potential fire hazard.
Another problem with employing aqueous HF compositions for etch cleaning is the slow rate of etching realized, possibly caused by deactivation of HF by water. To overcome this slow etch rate, most aqueous HF etching compositions need to incorporate at least 0.5% HF by weight. The slow etch rate of aqueous HF solutions can be of particular importance for MEMS devices. Silicon oxide dimensions in MEMS vary but are typically on the order of 1 .mu.m thick with lateral dimensions of 10-500 .mu.m. Slower etch rates lead to longer processing times. Etch assist holes are often added to polysilicon structures for which large, narrow regions of silicon oxide must be removed, such as for the release of micro-mirrors, in order to accommodate the slow etch rate of aqueous HF solutions and reduce etch times. The etch assist holes may adversely affect the ultimate device performance.