1. Field of the Invention
This invention relates to chemical processing and, more particularly, to systems and methods for monitoring attributes of multiple chemical mixtures using a single sensor, and further to systems and methods for controlling attributes of multiple chemical mixtures monitored by a single sensor, the systems and methods being especially configured for use in semiconductor processing.
2. Description of the Related Art
The information described below is not admitted to be prior art by virtue of its inclusion in this Background section.
There is much that separates the wide variety of activities that constitute the field of chemical processing. One common thread between chemical processes, however, is the extent to which variations in the attribute values of the chemical mixtures used in these processes can influence process outcomes. Chemical processes are typically designed on the assumption that values of chemical mixture attributes (e.g., temperature, concentration, particle count, and resistivity) can be maintained within predetermined ranges during processing. In many cases, these processes may be further designed on predictions of how chemical mixture attributes will vary within such ranges over time. Generally speaking, the success of a chemical process is dependent on how closely chemical mixture attributes can be maintained to ideal levels, with the degree of closeness necessary for success varying from industry to industry and from process to process. If, however, result-effective chemical mixture attributes are allowed to fall outside of predetermined ranges, a chemical process will often fail to achieve its goals.
One field of chemical processing where the importance of the above can be seen is in semiconductor processing. Chemical mixtures are used in a variety of ways in semiconductor processing, and play a particularly important role in etching and cleaning processes. Of the cleaning processes used in semiconductor processing, one of the most prevalent is the standard clean 1 (SC1) cleaning process. SC1 clean is the first step of the traditional RCA clean. SC1 solutions are solutions of water, hydrogen peroxide (H2O2), and ammonium hydroxide (NH4OH) (in order of decreasing typical concentration), and are often heated to between 60 and 85° C. during use. SC1 solutions may additionally include other chemicals, such as chelating agents used to bind up metallic ions present in the solution.
The SC1 clean may be used to remove residual organic and metallic contaminants that remain after various processing steps. For example, SC1 solutions are often used to remove residue remaining from chemical-mechanical polishing (CMP) procedures. CMP procedures are considered “dirty” procedures, and as such a great deal of residue often remains on wafer surfaces after such processes are complete. SC1 cleans may be used to remove this residue, reducing the defect probability in the finished product. In a typical post-CMP cleaning sequence, the wafers to be cleaned are immersed in a SC1 solution bath for a specified time to remove residue remaining from the CMP process. Alternately, spray-cleaning methods incorporating SC1 solutions may be used to clean the wafers.
When using SC1 solutions to clean silicon-bearing surfaces (e.g., a single-crystal silicon wafer and any polysilicon deposited thereupon), however, care must be taken to maintain concentrations of the component chemicals within certain values. While a properly balanced SC1 solution will not remove an inordinate amount of the silicon during cleaning, silicon surfaces can suffer chemical attack if concentrations deviate too severely from desired ranges during processing. For example, ammonium hydroxide will, in the absence of hydrogen peroxide, etch silicon. To avoid undesirable attack of silicon surfaces, it is important to maintain hydrogen peroxide concentration values within a SC1 solution at sufficiently high levels during processing.
Unfortunately, the hydrogen peroxide within an SC1 solution often decomposes over time. Such decomposition can occur, for example, as the result of impurities accumulating within the solution or if the solution temperature rises too far above desired levels. If an SC1 solution is used in which sufficient hydrogen peroxide is not present, the ammonium hydroxide can severely attack silicon surfaces, possibly reducing polysilicon feature sizes beyond acceptable levels or etching the wafer backside in such a way that problems are created further along in the manufacturing sequence. As linewidths reach 0.15 microns and below, it becomes even more important to maintain the component concentrations of an SC1 solution within acceptable ranges.
To prevent the occurrence of such undesirable situations, the concentration and other attributes of chemical solutions used in semiconductor processing have historically been checked using various qualification processes. Qualification processes attempt to characterize various parameters of a process, such as how solution properties change with continued use over time. For example, in one type of qualification process a series of test wafers may be sequentially sent through a chemical bath to determine how long the bath can be used in processing before one or more chemical concentrations (or other attributes) of the cleaning solution within the bath fall outside of acceptable ranges. From the data obtained in such a qualification process, the bath life may be estimated, and the overall cleaning sequence can be adjusted accordingly. However, such characterization techniques only estimate concentration from historical data, and as such are not always sensitive to the variations that may occur from run-to-run. In addition, when qualification wafers are being run through a process production wafers are not, so the extensive qualification of a process can reduce the total amount of time that process is available for use on production wafers.
In addition, other events often arise during processing for which qualifying processes are inadequate means of prevention. For example, chemical solutions used in semiconductor processing are often mixed from separate sources in a single tank during a process called pour-up. In a typical design, metering pumps are used to supply each constituent chemical of the solution being poured-up. The tanks also may include capacitive sensors to detect solution levels and control the balance of deionized water for pour-ups. In such configurations, the pour-up process may be an automated process that utilizes the metering pumps and capacitive sensors to mix a solution at a desired concentration. If not all the components of the pour-up system are operating optimally, however, bad pour-ups (i.e., situations in which the solution concentrations after pour-up are not at desired levels), can sometimes result. Even worse, one or more of these metering pumps can, from time to time, suffer total failure. Such metering pump failure may result in the chemical supplied from the failed metering pump being absent altogether from the final solution. What's more, metering pump failures are often not immediately detected, and may escape notice until the next qualification process is run. Processing a wafer with such an improperly balanced solution can cause irreversible damage, and if gone undetected, can even result in the loss of several lots of production wafers.
To avoid such problems, some manufacturers have resorted to placing concentration sensors on each tank supplying process chemicals to ensure that solution concentrations are within acceptable levels before the solutions are used in the processing of production wafers. Unfortunately, the concentration sensors required to reach desired accuracy levels are expensive, often costing $50,000 dollars or more. Furthermore, such sensors typically need to be rigorously maintained, further increasing their associated cost of ownership. In many cases, the cost of installing and maintaining concentration sensors on every chemical mixture that would profit from monitored outweighs the benefits obtained from concentration monitoring. Consequently, concentration monitoring may only be implemented in select tanks or forgone entirely.
Therefore, it would be desirable to design a monitoring system capable of monitoring an attribute (e.g., concentration) of multiple chemical mixtures within multiple chemical vessels during processing that did not require a separate concentration sensor for each chemical vessel being monitored.