The present disclosure relates generally to semiconductor plasma processing systems such as plasma reactors, and more specifically, to a gas distribution plate assembly for reducing recombination of species in a plasma environment, among others.
In the manufacture of integrated circuits, photolithography techniques are used to form integrated circuit patterns on a substrate, such as a silicon wafer. Typically, the substrate is coated with a photoresist, portions of which are exposed to activating radiation through a mask to image a desired circuit pattern on the photoresist. In the case of negative acting photoresists, the portions of the photoresist left unexposed to the activating radiation are removed by a processing solution, leaving only the exposed portions on the substrate. In the case of positive acting photoresists, the portions of the photoresist exposed to the activating radiation are removed by a processing solution, leaving only the unexposed portions on the substrate.
After subsequent processing, in which the integrated circuit components are formed, it is generally necessary to remove the patterned photoresist from the wafer. In addition, residue that has been introduced on the substrate surface through the subsequent processing such as etching must be removed. Typically, the photoresist is “ashed” or “burned” and the ashed or burned photoresist, along with the residue, is “stripped” or “cleaned” from the surface of the substrate. One manner of removing photoresist and residues is by rapidly heating the photoresist-covered substrate in a vacuum chamber to a preset temperature by infrared radiation, and directing a microwave-energized reactive plasma toward the heated substrate surface. In the resulting photoresist ashing process, wherein the reactive plasma reacts with the photoresist, the hot reactive gases in the plasma add heat to the surface of the substrate by means of convection. Heat energy on the order of 100 millliwatts per square centimeter (mW/cm2) is also added to the wafer as a result of the surface reaction. Excessive heat on the surface of the wafer can damage devices or portions thereof that have been formed on or in the wafer. In addition, excessive heat on the surface of the wafer can cause photoresist popping during subsequent processing, for example, during high-dose ion implanted (HDII) wafer ash processes, which could lead to unwanted particles on the wafer, causing device damage.
Reducing the temperature of the ashing process in the chamber will slow the reaction rate and thus the amount of heat added to the wafer by the surface reaction. Also, as the reaction rate slows, the rate of resist removal drops as well, leading to a reduction in throughput (defined as the number of wafers processed through the machine per unit of time). However, the gas temperature, which is a function of the gas mixture and the applied microwave power, will remain unaffected by the reduced process temperature. The problem is exacerbated if the process includes a reaction catalyst such as carbon tetrafluoride (CF4), which tends to increase the rate of reaction due to increased production of atomic oxygen. As a result, the catalyst-assisted process results in higher temperature gases, even at lower process temperatures.
A typical plasma processing apparatus is shown in U.S. Pat. No. 5,449,410 to Chang et al. wherein a baffle plate or showerhead is provided for distributing gas into a plasma chamber. The baffle plate is generally of a single or dual baffle plate configuration. An impingement plate may also be employed. The single baffle plate is generally fabricated from C-276 aluminum ultra pure alloy, which significantly adds to the costs of the reactor. The dual baffle plate generally includes a lower baffle plate fabricated from aluminum and an upper baffle plate fabricated from quartz, which may be further coated with a sapphire coating, which prevents the quartz from being etched in a fluorine environment.
Problems with the baffle plates are well known. For example, the sapphire coating on sapphire coated quartz baffle plates has a tendency to flake off during use requiring replacement. If left uncoated, the energetic species generated in the plasma, e.g., fluorine, can etch the baffle plate, thereby affecting flow patterns onto the substrate. In addition, the temperature of a quartz baffle plate can be difficult to control if IR wavelength energy is absorbed from the wafer with no means for sinking or dissipating the absorbed radiant energy. Impingement plates are generally formed of ceramic, which has a tendency to crack during use, leading to particle contamination as well as expensive preventative maintenance and repair schedule to properly clean the chamber.
Another problem with prior art baffle plates is with the relatively high gas flow rates required to produce effective ashing rates. For example, about 10 to about 12 standard liters per minute (slm) of total gas flow is typical for achieving effective ash rates and uniformity on 300 millimeter (mm) substrates. While the costs for gases such as oxygen and nitrogen are relatively inexpensive, costs for specialty gases become a concern at these high flow rates. Specialty gases, such as ultra high purity mixtures of hydrogen and some noble gas, are generally required for removing photoresist from substrates including low k dielectrics. The use of these gases can become cost prohibitive at these high gas flow rates.
Moreover, high gas flow rates tend to require very large pumps and vacuum lines to provide adequate chamber pressure. One of the reasons as to why high gas flow rates are employed is that while the plasma source upstream of the chamber generates a large number of reactive products, a majority of these species tends to recombine and release energy during the recombination. Recombination occurs in the volume of the plasma (and afterglow if the pressure is high enough) or on the surfaces of the chamber, the baffle plates, and plenums. Recombination changes the reactive characteristics of the plasma. For example, reactive atomic species that recombine into their molecular form become completely inert and non-reactive, thereby affecting the efficiency with which the ashing process is conducted.
Data taken by quartz microbalance indicate that atomic oxygen species at the wafer surface are on the order of about 1012 cm−3 at a pressure of about 1 Torr in the chamber. Since neutral density at that pressure is about 1016cm−3, the atomic oxygen concentration is about four orders of magnitude lower than the neutral concentration at the wafer surface. While the exact concentration of atomic oxygen is not known in the generation region, it can be presumed that a significant portion of the incoming oxygen gas flowing into the generator is dissociated into atomic oxygen, among other species, which becomes depleted as the species traverse through a plenum and one or two baffle plates to the substrate surface. This reduction is believed to be primarily due to the recombination of atomic oxygen on the baffle plate and chambers surfaces, as previously described.
Accordingly, there is a continuing need in the art for improvements to the plasma reactor, specifically by reducing the number of recombination sites within the plasma reactor while maintaining ash rate and non-uniformity of the process. The present disclosure is directed to a baffle plate configuration overcoming the problems noted in the prior art by reducing the number of recombination sites within the plasma reactor while maintaining ash rate and non-uniformity of the process. Because the number of recombination sites is reduced, lower gas flows can be employed while maintaining ash rate and non-uniformity of the process. This in turn could lead to a reduction in pump and vacuum hardware cost.