Patent Application
20080049201
The present invention relates to a lithographic apparatus and a method for cleaning a lithographic apparatus.
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in PCT patent application publication no. WO 99/49504. As illustrated in
A further immersion lithography solution with a localized liquid supply system is shown in
In European patent application publication no. EP 1420300 and United States patent application publication no. US 2004-0136494, each hereby incorporated in their entirety by reference, the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two tables for supporting a substrate. Leveling measurements are carried out with a table at a first position, without immersion liquid, and exposure is carried out with a table at a second position, where immersion liquid is present. Alternatively, the apparatus has only one table.
Immersion liquid may lift debris or particles (e.g. left over from the manufacturing process) from parts of the lithographic apparatus and/or substrates or erode components so as to introduce particles. This debris may then be left behind on the substrate after imaging or may interfere with the imaging while in suspension in the liquid between the projection system and the substrate. Thus, the issue of contamination should be addressed in an immersion lithographic apparatus.
It is desirable, for example, to provide a lithographic apparatus which can easily and effectively be cleaned as well as to provide a method for effectively cleaning an immersion lithographic apparatus.
According to an aspect of the invention, there is provided an immersion lithographic projection apparatus, comprising: a substrate table constructed and arranged to hold a substrate; a projection system configured to project a patterned beam of radiation onto the substrate; a megasonic transducer configured to clean a surface; and a liquid supply system constructed and arranged to supply liquid between the megasonic transducer and the surface to be cleaned.
According to an aspect of the invention, there is provided an immersion lithographic projection apparatus, comprising: a substrate table constructed and arranged to hold a substrate; a projection system configured to project a patterned beam of radiation onto the substrate; a megasonic transducer configured to clean a surface, the megasonic transducer being moveable relative to the surface such that, in a cleaning mode, a direct straight path through liquid exists between the megasonic transducer and the surface; and a liquid supply system configured to provide liquid between the transducer and the surface.
According to an aspect of the invention, there is provided a method of cleaning a surface of an immersion lithographic projection apparatus, comprising: covering at least a part of the surface to be cleaned in liquid; and introducing megasonic waves into the liquid.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:
an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation);
a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters;
a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PM configured to accurately position the substrate in accordance with certain parameters; and
a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.
The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.
The support structure holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.
The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.
As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more support structures). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
Referring to
The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.
The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in
The depicted apparatus could be used in at least one of the following modes:
1. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.
2. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.
3. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.
Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.
Another immersion lithography solution with a localized liquid supply system solution which has been proposed is to provide the liquid supply system with a barrier member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. Such a solution is illustrated in
The barrier member 12 at least partly contains liquid in the space 11 between a final element of the projection system PS and the substrate W. A contactless seal 16 to the substrate may be formed around the image field of the projection system so that liquid is confined within the space between the substrate surface and the final element of the projection system. The space is at least partly formed by the barrier member 12 positioned below and surrounding the final element of the projection system PS. Liquid is brought into the space below the projection system and within the barrier member 12 by liquid inlet 13 and may be removed by liquid outlet 13. The barrier member 12 may extend a little above the final element of the projection system and the liquid level rises above the final element so that a buffer of liquid is provided. The barrier member 12 has an inner periphery that at the upper end, in an embodiment, closely conforms to the shape of the projection system or the final element thereof and may, e.g., be round. At the bottom, the inner periphery closely conforms to the shape of the image field, e.g., rectangular though this need not be the case.
The liquid is contained in the space 11 by a gas seal 16 which, during use, is formed between the bottom of the barrier member 12 and the surface of the substrate W. The gas seal is formed by gas, e.g. air or synthetic air but, in an embodiment, N2 or another inert gas, provided under pressure via inlet 15 to the gap between barrier member 12 and substrate and extracted via outlet 14. The overpressure on the gas inlet 15, vacuum level on the outlet 14 and geometry of the gap are arranged so that there is a high-velocity gas flow inwards that confines the liquid. Those inlets/outlets may be annular grooves which surround the space 11 and the flow of gas 16 is effective to contain the liquid in the space 11. Such a system is disclosed in United States patent application publication no. US 2004-0207824, hereby incorporated in its entirety by reference.
As noted above, an immersion lithographic apparatus is one in which a substrate is imaged through liquid. That is, an immersion liquid is provided between a final element of the projection system PS and the substrate. This arrangement may pose one or more particular problems. In particular, the liquid should be confined in the apparatus and also the liquid should be kept as free as possible of foreign object particles which may cause defects during imaging and/or by being left on the substrate surface after imaging and prior to downstream processing. Sometimes the immersion liquid includes particles in suspension deliberately.
One or more embodiments of the present invention addresses the problem of foreign object particles by providing an apparatus and a method to clean an immersion lithographic projection apparatus in which a cleaning liquid is applied to the surface to be cleaned and megasonic waves are introduced into the liquid to clean the surface. The cleaning liquid may or may not be the same as the immersion liquid. It could, for example, be ultra pure water.
Compared to ultrasonic waves, megasonic waves produce cavitation bubbles (which implode or vibrate) which are very small and therefore may go very close to the surface to be cleaned. However, there is a limit to the amount of energy which may be introduced into the liquid using megasonics. Generally whereas ultrasonic energy may be introduced into a liquid anywhere and will be distributed throughout the liquid, megasonic energy is only locally high and must therefore be directed directly to the surface to be cleaned. That is, a direct path (line of sight/straight) must be present between the transducer introducing the megasonic waves and the surface to be cleaned. The whole length of that path should be filled with liquid.
Megasonic frequencies are generally regarded to be between 750 kHz to 3 MHz. For the present purposes frequencies of above about 750 kHz, above 1 MHz or above 1.5 MHz are used.
A stagnant boundary layer in the cleaning fluid near the surface of an object to be cleaned becomes thinner as frequency of the introduced sonic energy is increased. At megasonic frequencies cleaning is partly accomplished by acoustic streaming with high velocity pressure waves in the cleaning fluid as well as bubble vibration and, to a lesser extent cavitation and bubble bursting.
At megasonic frequencies, particles of less than 0.5 μm in diameter may be removed without damage to the surface being cleaned. As mentioned above, there must be a clear path from the transducer to the surface being cleaned (line of sight). In order to further increase the cleaning efficiency, gas may be dissolved into the liquid to promote cavitation (bubble formation). Suitable gases are nitrogen, carbon dioxide or oxygen but other gases may also be suitable such as ozone, or hydrogen (containing water). The use of a surfactant in the liquid can further enhance cleaning efficiency. Other possibilities to enhance cleaning efficiency include using a detergent or a solvent in the cleaning liquid or adding H2O2 solution.
Objects which one might want to clean in an immersion lithographic apparatus include, but not limited to, the substrate table WT which supports the substrate W (its top surface), the final element of the projection system PS which is immersed in immersion liquid during imaging and parts of the liquid confinement system (for example those illustrated in
In an embodiment, a liquid supply system is provided to provide liquid between a megasonic transducer and the surface to be cleaned. In the embodiment, the liquid supply system provides a flow of liquid so that liquid is removed as the surface is cleaned such that particles removed from the surface are carried away. One suitable liquid is water in an ultra pure form. However, other types of liquid may be suitable. Furthermore, an addition to the liquid such as a surfactant as mentioned above may also have an advantage. Other cleaning liquids are water/hydrogen peroxide, water/ethanol, water/iso-propylalcohol (IPA), water/ammonia or water/acetone mixtures, for example. Other chemicals which may be useful as an addition include TMAH and SC-1 or SC-2.
One reason for introducing gas (or some solvents) into the liquid is that this promotes stable cavitation. This results in stable bubbles being formed in the liquid. These bubbles are then vibrated by the megasonic waves which results in cleaning which is likely to do less damage to the surface being cleaned than so called transient cavitation which is cavitation where a solvent evaporates into a bubble and then implodes or collapses. These violent implosions may lead to damage of the surface and are typically seen at ultrasonic frequencies and are less significant at megasonic frequencies where the bubbles produced tend to be smaller than those produced at ultrasonic frequencies. However, as noted above, the megasonic waves need to be supplied in line of sight of the surface they are to clean.
A process time up to 100 seconds may lead to a particle removal efficiency of up to 100% at a frequency of about 1 MHz. If the acoustic frequency exceeds much more than 3 MHz the particle removal efficiency is dramatically decreased over a frequency of just above 1 MHz. The introduction of gas into the liquid has a major effect on the particle removal efficiency. Removal of 34 nm diameter SiO2 particles may increase from zero removal efficiency to 30% removal efficiency with the introduction of oxygen at a level of 20 ppm into the liquid. Thus, a gas concentration of above about 5 ppm can be useful.
Temperature may also be important and a balance should be drawn between faster reaction time at a high temperature (say 55° C.) over less gas being dissolved at a high temperature.
There is also an effect of the pH of the liquid. At low pH, there are many H+ ions in the liquid which results in a positive surface charge. Similarly, at high pH the liquid contains many OH ions which results in a negative surface charge. Therefore ensuring that the pH of the liquid is distant from pH 7 ensures that re-deposition of particles after they have been removed does not occur. Moreover, the electrostatic repulsion between the particle and the surface when both are charged equally (either positively or negatively) assists in lifting the particle from the surface.
The power of the transducer should be between 0.2 and 5 W/cm2, the irradiation distance should be between 5 and 20 mm and a cleaning time should be between 10 and 90 seconds. For the acoustic waves from the megasonic transducer to travel a direct path to the surface to be cleaned from the megasonic transducer, several designs are proposed to clean different parts of the immersion lithographic apparatus.
A first embodiment is illustrated in
The substrate table WT and/or the barrier member 12 and transducer 20 are moved relative to the other so that all of the top surface of the substrate table WT may be cleaned. The cleaning could take place within the lithographic apparatus in an automated way or could be carried out by manually bypassing the barrier member 12 and transducer 20 over the top surface of the substrate table WT by hand or by some tooling.
If the cleaning process is automated, one way of arranging for this is to provide the barrier member 12 and transducer 20 arrangement to be moveable from a stationary store position to a (stationary) clean position and to move the substrate table WT relative to the transducer 20 when the transducer is in the (stationary) clean position. The substrate table WT may need to be moved in the Z axis prior to activation of the liquid supply system of the cleaning device. Once the gas flow 16 has been created the barrier member 12 is supplied with liquid and the substrate table WT is moved in the X-Y plane so that the surfaces which are desired to be cleaned can be cleaned.
In this embodiment a moveable bath 50 is provided which has on its bottom surface a megasonic transducer 20. The megasonic transducer 20 of the second embodiment may be larger than the megasonic transducer of the first embodiment because there are no particular size constraints. Thus, the distance between the megasonic transducer and the surface to be cleaned (the bottom surface of the liquid confinement system) can be made larger and up to 50 mm. In an embodiment, the distance is less than 40 mm or less than 30 mm.
A controller controls the position of the bath 50 which is moveable between a store position and a cleaning position (as illustrated) and which controls the level of fluid in the bath 50. In one embodiment the level of liquid is controlled so as to cover the bottom surface of the liquid confinement system LCS but not to cover the final element of the projection system PS. Thus, the gap 75 is not filled with liquid. This is to protect the final element of the projection system from the sonic waves and/or the cleaning liquid which could damage it. Additionally or alternatively, a shield (perhaps in the form of a plate) may be used to shield the final element of the projection system from megasonic waves and/or liquid. In an embodiment, this bath arrangement may be used to clean the final element of the projection system PS and in which case the controller increases the level of liquid in the bath 50 so that the final element of the projection system PS is covered in liquid.
In the second embodiment illustrated in
In a third embodiment illustrated in
When the top surface of the substrate table WT is cleaned, it is also possible to clean at the same time one or more sensors provided on the top surface of the substrate table WT. Examples of types of sensors include a transmission image sensor, a lens interferometer, and/or a spot sensor.
In an embodiment, it may be useful to ensure that the acoustic waves produced by the transducer impinge at 90° onto the surface to be cleaned. For this purpose, a micrometer may be provided to adjust the tilt of the transducer 20 relative to the surface to be cleaned. In an embodiment, it may be advantageous to provide the transducer tilted relative to the surface and again, this can be adjusted using a micrometer. A micrometer may also be used to adjust the distance from the transducer to the surface to be cleaned. In all embodiments, a flow of liquid across the distance between the transducer and the surface to be cleaned is desirable though not essential.
The above described megasonic cleaner is well suited to removing particles from a surface. However, once those particles have been removed they sometimes re-attach themselves to a surface unless the liquid in which the particles are suspended is quickly moved away. Therefore, it is desirable to provide a flow of liquid between the megasonic transducer and the surface being cleaned using, for example, a pump. In particular, it is desirable to design the apparatus such that there are no or few locations of zero flow velocity (stagnant zones).
Another way to prevent or at least reduce re-attachment of the particles to the surface is to change one or more properties of the liquid between the megasonic transducer and the surface to ensure that the zeta potential of the particles and the zeta potential of the surface is such that the particles are not attracted to the surface, desirably such that they are repelled from the surface.
The zeta potential is the potential of a surface in a liquid. The zeta potential generally decreases with distance from the surface. A given type of material has a given zeta potential for a particular type of liquid. One way of varying the zeta potential of a surface is to change the concentration of electrolyte in the liquid and another method to change the zeta potential is to change the pH of the liquid. By careful selection of the concentration of electrolyte in the liquid (e.g. salt) or the pH of the liquid, (i) the zeta potential of the surface from which the particles are removed (and/or any other surface where adherence is to be avoided) and (ii) the zeta potential of the particles can be chosen. Desirably those two zeta potentials are chosen such that they have the same polarity and thereby repel.
The pH of the liquid and/or concentration of the electrolyte is chosen with a knowledge of the material from which the surface which is being cleaned is made and with a knowledge of the type of material the particles are likely to be made. If the materials are the same, then it should easy to select a pH or electrolyte concentration at which the zeta potential is non zero for both the surface and the particle. In that circumstance the potential would be either positive or negative for both the surface and the particle such that they would repel one another and the particles would be unlikely to re-adhere to the surface. If the materials are different, pH or electrolyte concentration may be harder to choose, but it is likely that there will be at least one pH and/or concentration at which the zeta potential will have the same polarity for both materials. A controller may be provided to control the pH of the liquid and/or concentration of electrolyte in the liquid based upon one or more aspects of the foregoing knowledge.
Because changing the pH of the liquid may have a negative impact on the solubility of materials, which can itself result in contamination or loss of material integrity, it is desired to change the electrolyte concentration. If this is done by adding salt (NaCl) then this does not greatly affect the pH of the liquid (i.e. the liquid remains substantially neutral).
The use of any or all the above techniques in combination (in particular changing pH and electrolyte concentration to change zeta potentials and the use of a surfactant) may be the best approach.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm).
The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive and reflective optical components.
While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein.
One or more embodiments of the invention may be applied to any immersion lithography apparatus, in particular, but not exclusively, those types mentioned above and whether the immersion liquid is provided in the form of a bath or only on a localized surface area of the substrate. A liquid supply system as contemplated herein should be broadly construed. In certain embodiments, it may be a mechanism or combination of structures that provides a liquid to a space between the projection system and the substrate and/or substrate table. It may comprise a combination of one or more structures, one or more liquid inlets, one or more gas inlets, one or more gas outlets, and/or one or more liquid outlets that provide liquid to the space. In an embodiment, a surface of the space may be a portion of the substrate and/or substrate table, or a surface of the space may completely cover a surface of the substrate and/or substrate table, or the space may envelop the substrate and/or substrate table. The liquid supply system may optionally further include one or more elements to control the position, quantity, quality, shape, flow rate or any other features of the liquid.
The immersion liquid used in the apparatus may have different compositions, according to the desired properties and the wavelength of exposure radiation used. For an exposure wavelength of 193 nm, ultra pure water or water-based compositions may be used and for this reason the immersion liquid is sometimes referred to as water and water-related terms such as hydrophilic, hydrophobic, humidity, etc. may be used.
The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.
This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 11/437,876 filed May 22, 2006, the entire contents of which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11437876 | May 2006 | US |
| Child | 11802082 | May 2007 | US |
