Patent Application
20070062372
This invention relates generally to method and apparatus for producing a mixture of an oxidizer and a high pressure fluid useful for cleaning objects such as integrated circuit wafers and for disinfecting food or water and particularly to method and apparatus for producing a mixture of ozone and supercritical or high pressure carbon dioxide (SCCO2 or HPCO2) useful for cleaning objects and for disinfecting food or water.
Cleaning objects prior to performing work on them is an essential step in many manufacturing processes. One manufacturing process will be discussed in detail. For example, semiconductor integrated circuit manufacture has many steps in which a pattern is transferred from a mask to a substrate. The pattern is typically transferred by selective exposure of the substrate to radiation through a mask. The substrate is coated with a radiation sensitive material, termed a resist, whose solubility when exposed to an appropriate developer is altered by the radiation. After selected portions of the resist are removed, the now exposed portions of the substrate are modified by, for example, ion implantation, etching as well as other processes. After the modification is complete, the resist is removed and the process repeated until integrated circuit fabrication is complete.
As can be readily appreciated, the pattern must be accurately transferred from the mask to the substrate and this requires complete removal of the resist, as well as any unwanted material remaining from the process step, before the resist for the next process step is deposited and covers the substrate. Resists have typically been removed, that is, stripped, by either a wet technique, such as a HF rinse or a dry technique such as ashing. The latter technique essentially burns off the resist in an oxygen plasma. Although adequate for many purposes, these techniques have been found to possess drawbacks now that device dimensions are in the submicron regime. There are at least two potential problems. First, there may be unwanted debris remaining with dimensions comparable to device dimensions. Second, resist removal may be incomplete. It has been found that some process steps, for example, dry etching, may harden a portion of the resist and render it impervious to conventional stripping techniques. Accordingly, techniques other than the wet and dry techniques previously mentioned have been examined to determine their suitability for use in integrated circuit manufacture.
Another cleaning technique uses supercritical fluids as a solvent for unwanted particles. A supercritical fluid is a material that is above both its critical temperature, Tc, and critical pressure, Pc. These values define the highest temperature and highest pressure at which the vapor and liquid phases of the material can exist in equilibrium and thus define the critical point. The critical point can be understood by considering what happens physically along the line separating the liquid and vapor phases as both pressure and temperature are increased. The gas density increases and the liquid density decreases due to thermal expansion. When the two densities are equal, a supercritical fluid is present. Both temperature and pressure may be further increased from the critical point with the material remaining a supercritical fluid.
One supercritical fluid that has been examined for cleaning processes is supercritical carbon dioxide (SCCO2). This material is attractive for use as a cleaning agent because it has a solubility comparable to those of light hydrocarbons without their environmental problems, and it has a relatively low surface tension. The latter attribute facilitates cleaning of small dimension features, such as holes in a semiconductor substrate, because the SCCO2 can enter and clean the hole more easily than can high surface tension fluids.
The literature describing the use of SCCO2 for cleaning is now extensive. For example, U.S. Pat. No. 6,602,349 describes the use of SCCO2, with or without additives including solvents and surfactants, in cleaning semiconductor wafers to remove photoresist. U.S. Pat. No. 6,602,351 also teaches the use of SCCO2 together with a solvent or surfactant for cleaning semiconductor surfaces. In addition to semiconductor integrated circuit wafers, mention is made of cleaning other devices such as micro-electro-mechanical and opto-electronic devices.
A further cleaning technique uses ozone, a strong oxidizing agent, to remove unwanted resist. The use of ozone for cleaning semiconductor wafers is described in United States Patent Application Publication 2002/0157686, wherein a layer of heated liquid, for example, water or HF, covers the wafer, then ozone is provided and diffuses through the liquid. The ozone reacts with unwanted material, such as photoresist, and thus facilitates its removal.
U.S. Pat. No. 5,507,957 describes another use of ozone, namely, the treatment of fluids. Disinfecting water or food, for example, juice, may be considered to be a type of cleaning as unwanted entities are removed or rendered harmless. For example, enzymes, which cause spoilage, are destroyed. As a pure or purer product results, this process may also be thought of as a manufacturing or cleaning process. In the treatment described, ozone containing oxygen is passed through a first adsorbing bed which preferentially adsorbs ozone. The nonadsorbed oxygen rich gas and air are passed through a second adsorbing bed which preferentially adsorbs nitrogen. Subsequently, the adsorbed ozone and nitrogen are desorbed and the combined stream then contacts the material being treated.
U.S. Pat. No. 6,242,165 describes a method for cleaning organic material from semiconductor wafers using an oxidizer in a supercritical state. Oxidizers include supercritical SO3, supercritical H2O2, supercritical O2, and supercritical O3. The cleaning composition optionally includes supercritical components such as CO2 or inert gases that are mixed in a mixing manifold.
While it is desirable to mix ozone from an ozone generator, the ozone being at a low pressure, with a fluid such as SCCO2, which is at high pressure, such mixing of fluids at different pressures is generally difficult and additional apparatus and methods for forming a mixture of SCCO2 and ozone are desirable.
One embodiment of the present invention relates to an apparatus comprising an adsorption bed, an oxidizer source connected to the adsorption bed wherein the oxidizer is at a first pressure, a high pressure fluid source connected to the adsorption bed wherein the high pressure fluid is at a second pressure, the second pressure being greater than the first pressure, a depleted oxidizer outlet, and a fluid mixture outlet comprising a mixture of oxidizer and high pressure fluid.
According to another embodiment of the present invention, the apparatus includes a first and a second adsorption bed, an oxidizer source connected to the adsorption beds wherein the oxidizer is at a first pressure, a high pressure fluid source connected to the adsorption beds wherein the high pressure fluid is at a second pressure, the second pressure being greater than the first pressure, a depleted oxidizer outlet connected to the adsorption beds, and a fluid outlet comprising a mixture of oxidizer and high pressure fluid.
One method according to the present invention comprises adsorbing an oxidizer in an adsorption bed, desorbing the oxidizer by adsorbing a high pressure fluid in the adsorption bed, producing an outlet fluid mixture of oxidizer and high pressure fluid, and directing the outlet fluid mixture to a device.
The depleted oxidizer flowing through oxidizer outlet 225 may be recycled through a recycle system or exhausted to an exhaust waste treatment system. The high pressure fluid and oxidizer mixture flowing through fluid mixture outlet 227 may flow directly to a device or tool, such as cleaning chamber 103 shown in
The operation of the apparatus shown in
When the oxidizer concentration, as measured by the oxidizer sensor, reaches a predetermined setpoint, oxidizer flow through the adsorption bed 201 will stop and a high pressure fluid from high pressure fluid source 207, such as supercritical carbon dioxide, will begin to flow through adsorption bed 201. The high pressure fluid adsorbs onto the adsorbent thereby displacing the previously adsorbed oxidizer. This in turn, creates a mixture of the oxidizer and high pressure fluid that flows through fluid mixture outlet 227 and to a device such as a semiconductor processing chamber or a storage vessel.
A further sensor may be associated with the fluid mixture outlet 227 and connected to a programmable logic controller (PLC) to monitor the oxidizer concentration in the fluid mixture. In addition, a flow controller for controlling the flow rate of high pressure fluid into the adsorption bed 201 may be fluidly connected to high pressure fluid source 207 and electrically connected to the PLC. The operation of the apparatus in this configuration would enable monitoring of the oxidizer concentration in fluid mixture outlet 227 with the sensor and providing a signal indicative of oxidizer concentration in the fluid mixture to the PLC. The PLC would then send a signal to the flow controller to adjust the high pressure fluid flow rate based upon a predetermined setpoint for the desired oxidizer concentration in the fluid mixture exiting from the fluid mixture outlet 227.
As an optional step, following desorption of oxidizer, the bed is vented to the atmosphere, and high pressure fluid in the void space and any remaining in the adsorption bed 201 is removed by flowing a purge gas, such as oxygen, through the adsorption bed 201. The oxidizer adsorption, desorption with high pressure fluid and high pressure fluid removal steps are repeated cyclically until cleaning is complete. The process as described with respect to
In another embodiment of the present invention, shown in
The operation of the apparatus shown in
The cycle time for the dual adsorption bed process is preferably in the range between 2 and 20 minutes. For example, time periods for the various steps according to one embodiment of the present invention are summarized in the following table.
The cycles may be operated continuously with the appropriate valves being opened and closed as steps begin and stop during the cycle, as is known in the art.
There are several alternatives available for valve control systems for operating the apparatus and performing the methods of the present invention. For example, valves may be controlled with a computer, mechanically or even manually. Further, different valves may be controlled in different manners.
The oxidizer source for the system of the present invention can be an ozone generator, which produces a mixture of oxygen and ozone (O2 and O3) by partially converting a stream of oxygen into the ozone. The appropriate amount of conversion is set according to the desired outcome, and the highest ozone concentration is not always used because higher concentrations require more power to generate and thus have a higher cost. A practical maximum concentration is 20 percent O3. An oxygen rich feed gas for producing the oxidizer, such as ozone, may be produced from a pressure swing adsorption (PSA) facility. While ozone is the preferred oxidizer, other oxidizers may be used, such as hydrogen peroxide (H2O2) or nitrogen trifluoride (NF3).
The high pressure fluid is typically chosen based on the material being cleaned. For semiconductor cleaning, SCCO2 is usually preferred, while for disinfecting food products such as juice or drinking water, high pressure CO2 including SCCO2 may be used. The high pressure fluid may optionally contain co-solvents such as alcohols or disinfectants. The high pressure fluids and their generation are well known in the art.
Suitable adsorbents for the adsorption beds include silica gel, high silica mordenites and other materials that do not destroy ozone to a significant extent during adsorption. The appropriate adsorbents for the adsorption beds may be chosen by the operator based on the high pressure fluid and oxidizer used.
The operating parameters for the system according to the present invention can be readily set by the operator skilled in the art. For example, the adsorption beds are sized to adsorb the desired amount of fluid. A useful range for oxidizer adsorption pressures is from 5 psig to 50 psig (pounds per square inch gauge) because it approximately matches the pressure of the ozone/oxygen mixture from oxidizer. The desorption pressure using high pressure fluid is preferably in the range of 50 psia to 4000 psia (pounds per square inch absolute). In a more particular example, when treating water, the pressure range is typically between 50 psia and 200 psia. When using ozone as the oxidizer, the ozone concentration may be varied between 6 percent and 20 percent and the flow rate of the high pressure fluid may also be varied.
There are several alternatives that may improve the cycle times. For example, the purge gas may be used at the same temperature as the oxidizer feed, however, a slightly higher temperature, for example, 10 to 30 degrees C. higher than the feed temperature, for the purge gas during part of the purge may reduce the amount of purge gas needed. Standard heaters may be used to heat the purge gas. Additionally, an ozone compatible vacuum pump, for example, a dry vacuum or a water ring vacuum, may be used to reduce the amount of purge gas required during the purge operation.
While
Moreover, while it is understood that the term oxidizer includes such standard oxidizers as ozone, hydrogen peroxide, and nitrogen trifluoride, in food and water disinfecting applications where ozone is used, the ozone may react with enzymes or microorganisms by mechanisms other than oxidation. Hence, the term oxidizer is here defined to embrace ozone when employed in food and water disinfecting applications.
Other variations in the apparatus and operation are contemplated. For example, more than two adsorption beds may be used. Moreover, as noted above, additional solvents or disinfectants may be added to the high pressure fluid.
It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description and examples, and it is intended that such embodiments and variations likewise be included within the scope of the invention as set out in the following claims.
