System and method for monitoring contamination Number:7,092,077 from the United States Patent and Trademark Office (PTO) owispatent The present invention provides passive sampling systems and methods for monitoring contaminants in a semiconductor processing system. In one embodiment, that passive sampling system comprises a collection device in fluid communication with a sample line that provides a flow of gas from a semiconductor processing system. The collection device is configured to sample by diffusion one or more contaminants in the flow of gas.<H contamination, monitoring, System, and, meth, 7092077.html, Patent, pto, 7092077

Title: System and method for monitoring contamination

Abstract: The present invention provides passive sampling systems and methods for monitoring contaminants in a semiconductor processing system. In one embodiment, that passive sampling system comprises a collection device in fluid communication with a sample line that provides a flow of gas from a semiconductor processing system. The collection device is configured to sample by diffusion one or more contaminants in the flow of gas.

Patent Number: 7,092,077 Issued on 08/15/2006 to Kishkovich, et al.

Inventors: Kishkovich; Oleg P. (Greenville, RI), Grayfer; Anatoly (Newton, MA), Goodwin; William M. (Medway, MA), Kinkead; Devon (Holliston, MA)
Assignee: Entegris, Inc. (Chaska, MN)

Appl. No.: 10/662,892

Filed: September 15, 2003

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
10395834	Mar., 2003
10253401	Sep., 2002	6759254
09961802	Sep., 2001	6620630

Current U.S. Class: 356/36 ; 438/14

Current International Class: G01N 1/00 (20060101); H01L 21/66 (20060101)

Field of Search: 257/66,629,687,700,713,78-80,99 250/306,440.11,441.11,442.11,443.11,336.1,338.5,305,338,339,343,345,373,504,339.01,370.01,374 438/7,14,16,102,115,101,166,22,365,482,509,513,540,57,597,603,618,680,635,783-784,795 356/246,432,437,499

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Primary Examiner: Lebentritt; Michael
Assistant Examiner: Stevenson; Andre'
Attorney, Agent or Firm: Weingarten, Schurgin, Gagnebin & Lebovici LLP

Parent Case Text

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patent application Ser. No. 10/395,834 filed Mar. 24, 2003, which is continuation-in-part of U.S. patent application Ser. No. 10/253,401, filed Sep. 24, 2002 now U.S. Pat. No. 6,759,254, which is a continuation-in-part of U.S. patent application Ser. No. 09/961,802, filed Sep. 24, 2001 now U.S. Pat. No. 6,620,630. The entire contents of the above applications are incorporated herein by reference.

Claims

What is claimed is:

1. A system for determining and monitoring contamination in a photolithography instrument, comprising at least one collection device in fluid communication with a gas flow extending through an optical system of the photolithography instrument, the collection device having an adsorptive material and a saturation capacity for a lower molecular weight contaminant, the collection device being operated past the saturation capacity of the lower molecular weight contaminant while continuing to adsorb higher molecular weight contaminants in the gas flow.

2. The system of claim 1, wherein the adsorptive material comprises glass spheres having predetermined surface properties for adsorption of contaminants.

3. The system of claim 1, wherein the collection device is tubular.

4. The system of claim 1, further comprising a collection device that is not in fluid communication with the gas flow.

5. The system of claim 1, wherein the collection device is at least one of glass and coated glass material.

6. The system of claim 1, wherein the adsorptive material comprises the polymer Tenax.RTM..

7. The system of claim 1, wherein the contamination includes at least one of refractory compounds, high molecular weight compounds and low molecule weight compounds.

8. A contamination analysis apparatus in a photolithography system having an optical element comprising: a collection device comprising a first material having a surface property of the optical element coupled to a gas flow, the collection device being coupled to a light source such that light is optically coupled to a surface of the material and having an adsorptive material with a saturation capacity to adsorb contaminants in the gas flow.

9. The contamination analysis apparatus of claim 8, wherein the adsorptive material comprises a polymer that absorbs higher molecular weight organic compounds.

10. The contamination analysis apparatus of claim 8, wherein the adsorptive material comprises glass spheres.

11. The contamination analysis apparatus of claim 8, wherein the contaminants include at least one of refractory compounds, high molecular weight compounds and low molecular weight compounds.

12. A filtering system for removing contamination in a semiconductor processing system, comprising at least one collection device in fluid communication with a gas flow extending through an optical system of the semiconductor processing system, at least one collection device having a selectively permeable membrane that filters contaminants including a refractory compound, a high molecular weight compound and a low molecular weight compound from the gas flow, the collection device being operated past a saturation of the membrane to absorb the low molecular weight compound.

13. The filtering system of claim 12, wherein the collection device is coupled to a vacuum source to increase a pressure gradient across the selective membrane.

14. The filtering system of claim 12, wherein the gas flow comprises clean dry air, nitrogen, and/or other inert gases.

15. The filtering system of claim 12, further comprising a regenerative adsorption device in fluid communication with an output permeate stream from the selectively permeable membrane.

16. The filtering system of claim 12, further comprising a second collection device in fluid communication with a residue stream of the collection device, the second collection device having a second membrane that is selectively permeable to oxygen and water.

Description

BACKGROUND OF THE INVENTION

Semiconductor manufacturers continue to measure and control the level of contamination in the processing environment, especially during the critical steps of the photolithography processes. The typical means of determining the quality and quantity of contamination in gas samples in cleanroom manufacturing environments involves sampling air and purge gases, such as, for example, filtered and unfiltered air, clean dry air, and nitrogen, with sampling tubes or traps, typically containing adsorptive medium such as, the polymer Tenax.RTM.. This sampling process is followed by analysis using thermal desorption, gas chromatography and mass spectrometry (TD/GC/MS). The combination of TD/GC/MS provides identification of sample components and a determination of the concentration of these components. The most abundant contaminants in these manufacturing environments are low molecular weight components such as acetone and isopropyl alcohol. The current sampling time for existing traps typically varies between 0.5 and 6 hours with total accumulated sample volumes ranging typically between 20 and 50 liters.

Further, in applications that are directed to the manufacturing of or use of optical elements such as, for example, photolithography, the detection and quantification of compounds having a higher molecular weight such as, for example, siloxanes is of primary concern. These compounds having a higher molecular weight are, however, typically in much lower concentrations as compared with the low molecular weight species. Further, the compounds having a high molecular weight can also be defined as condensable compounds with a boiling point typically greater than approximately 150.degree. C. The current methods for determining contamination have the limitation of the sample volume being based on the total trap capacity of the lighter or lower molecular weight components, for example, compounds having typically less than six carbon atoms. As the heavier components are usually present at much lower concentrations, the collection of a significant mass of these higher molecular weight species is limited.

In addition, polluting or contaminating substances may adhere onto the optical elements and reduce the transmission of light. Currently airborne contamination is addressed in cleanroom environments with little regard for contaminants that may be adsorbed onto the surfaces of optical elements. The adsorbed contamination reduces the transmission of light through the optical elements and system.

Thus, contamination of optical systems is emerging as a significant risk to photolithography and other semiconductor manufacturing processes as shorter wavelengths of the electromagnetic spectrum are exploited. However, molecular films on optical surfaces physically absorb and scatter incoming light. Scattered or absorbed light in photolithography optical surfaces causes distortion of the spherical quality of wavefronts. When the information contained in the spherical wavefront is distorted, the resulting image is also misformed or abberated. Image distortions, or in the case of photolithography, the inability to accurately reproduce the circuit pattern on the reticle, cause a loss of critical dimension control and process yield.

Typically, filter systems are used to remove molecular contamination in semiconductor processing environments. Systems are in place to measure the performance of such filter systems. However, typical monitoring of filter performance includes measurement of filter breakthrough either by process failure or by detection of the target filtered gas at the discharge of the filter system. However, these measurement means detect breakthrough after it has occurred.

A need still exists for determining, accurately and efficiently, the presence and quantity of contaminants that can alter and degrade the optical systems in semiconductor processing instruments. There further remains a need to monitor the performance of gas phase filter systems prior to a breakthrough failure.

SUMMARY OF THE INVENTION

The preferred embodiments of the system of the present invention provide an accurate and efficient system of determining and/or controlling the quality and/or quantity of contamination within a gas sample which can reduce the performance of optical elements used in semiconductor processing instruments, such as, for example, within the light path of a deep ultraviolet photolithography exposure tool. In a preferred embodiment of the present invention, the contamination may be gaseous as well as contamination adsorbed onto optical surfaces. Optical performance can be evaluated without limitation as the level of transmitted or reflected light through an optical system. The embodiments of the system and method of the present invention are predicated on the recognition that compounds having both high and low molecular weights can contribute to the contamination of optical systems but can operate at different rates. As such, the contaminants that negatively impact the performance of optical elements can be described in terms of different orders, such as, for example, first, second and third order effects.

First and second order contaminating effects have a greater impact on contamination of optical systems than third or fourth order contaminants. The first order contaminants may comprise high molecular weight organics such as, for example, C.sub.6 siloxanes and C.sub.6 iodates with an inorganic component which is not volatilized through combination with oxygen. Second order contaminants may comprise high molecular weight organics, such as, for example, compounds including carbon atoms within the range of approximately six to thirty carbon atoms (C.sub.6 C.sub.30). Third order effects can arise due to the contaminating effects of organics such as C.sub.3 C.sub.6 that have approximately three to six carbon atoms. Fourth order contaminants include organics such as, for example, methane, that have approximately one to five carbon atoms. In many applications, the first and second order contamination can have a much lower concentration than the third and/or fourth order contamination, yet have a significantly greater effect on the operation of the system.

A preferred embodiment in accordance with the present invention of a method for detecting and monitoring, and preferably removing contamination in a semiconductor processing system includes delivering a gas sample from the processing system to a collection device. The method further includes collecting contamination which comprises refractory compounds, and high and low molecular weight compounds, from the gas in the collection device by sampling the gas for a duration exceeding the saturation capacity of the collection device for high molecular weight compounds. The compounds having a high molecular weight are condensable with a boiling point typically greater than approximately 150.degree. C.

A preferred embodiment of the system and method of the present invention for determining contamination includes the detection of refractory compounds such as, for example, siloxanes, silanes and iodates, and high molecular weight organics. The preferred embodiment includes the removal of refractory compounds, high molecular weight organics and low molecular weight organics, all of which contribute to the contamination of optical systems, but which can operate at different contamination rates.

The system of the present invention for determining contamination can use different types of sample collecting media. In a preferred embodiment, the sample collecting media can emulate the environment of the optical surfaces of interest such as, for example, the absorptive or reactive properties of the optical surfaces. A measure of contamination adsorbed onto optical surfaces enables the minimization and preferably the removal of the contaminants. In another preferred embodiment, a polymer that has a high capacity for absorbing the compounds with a high boiling point is used in a collection device such as, for example, Tenax.RTM. a polymer based on 2 6 diphenyl p-phenylene. The operation of the system in accordance with a preferred embodiment of the present invention includes quantitatively measuring the concentration of both low and high boiling point compounds in the same sample wherein the collection device has been driven beyond the breakthrough volume or saturation capacity of the collection media to capture the low molecular weight compounds. The breakthrough volume of the collection device is defined in a preferred embodiment as the quantity of gas needed to go beyond the adsorption capacity of the device.

In accordance with a preferred embodiment of the present invention, the method for detecting contamination includes a sampling time extended by, for example, a number of hours, days or weeks to enable collection of an appropriate mass of contaminants which are present in relatively low concentration. In a preferred embodiment, the sampling time is typically beyond the breakthrough capacity of the collection device for low molecular weight components, is at least six hours long and preferably within a range of six to twenty-four hours for a sampling tube system. The extended time allows for the collection of higher masses of refractory compounds and higher molecular weight compounds that may interfere with the performance of optical components even more than low molecular weight compounds. The higher molecular weight compounds include, but are not limited to, for example, siloxanes and silanes.

In accordance with another preferred embodiment of the present invention, a semiconductor processing instrument, for example, a photolithography cluster, includes a filtering system to remove contaminants. The filtering system includes a selective membrane to filter organic compounds from a gas stream.

A preferred embodiment includes a method for monitoring the performance of a filter positioned in an airstream in a semiconductor processing system. The method includes sampling the airstream at a location upstream of the filter to detect the molecular contaminants present in the airstream, identifying a target species in the contaminants upstream of the filter, selecting a non-polluting species of a contaminant having a concentration greater than a concentration of the target species, measuring the non-polluting species in the airstream at a plurality of locations, and determining the performance of the filter with respect to the target species from measurements of the non-polluting species. The plurality of locations includes, but is not limited to, a location downstream of the filter and at a location within the filter. Further, the method for monitoring includes generating a numerical representation of a chromatogram of the airstream sampled at a location upstream of the filter. The method for monitoring includes the non-polluting species having a molecular weight that is lower than that of the target species. A correlation is established between the low and high molecular weight compounds. In addition, in the method for monitoring, the step of sampling includes collecting refractory compounds, high molecular weight compounds and low molecular weight compounds. The filter comprises absorptive material.

A preferred embodiment includes a system for determining and monitoring contamination in a photolithography instrument, having at least one collection device in fluid communication with a gas flow extending through an optical system of the tool, the collection device having a material analogous to optical elements, and a light source providing high energy light to the collection device such that at least one contaminant in the gas flow reacts with the light to create a deposition layer on the material. Further, the system includes at least one photodetector coupled to the collection device to detect the presence of the deposition layer on the material by monitoring either the spectral or transmission differences. The material in the system comprises glass spheres having predetermined surface properties for adsorption of contaminants. The material is at least one of glass and coated glass material. The contamination includes at least one of refractory compounds, high molecular weight compounds and low molecular weight compounds.

In accordance with another aspect of the present invention, an apparatus for determining contamination in a semiconductor processing system includes a filter system having a plurality of filter traps for collecting contaminants from a gas stream for a duration, and an interface module coupled to the filter system in fluid communication with a gas flow extending through the processing system and directing a portion of the gas flow into and out of the filter system.

The contaminants include at least one of refractory compounds, high molecular weight compounds and low molecular weight compounds. A vacuum source can be coupled to the filter system to increase a pressure gradient across the filter traps. The filter traps can have a permeable membrane that filter contaminants such as at least one of a refractory compound, a high molecular weight compound and a low molecular weight compound from the gas flow.

In preferred embodiments, the interface module further comprises a pressure regulation device, a controller, electronically controlled valves to impose a duty cycle for sampling, a timer device to determine a sampling duration and a cooling device such as a thermoelectric cooling device. Further, the filter traps have an absorptive material such as a polymer, for example, Tenax.RTM..

The foregoing and other features and advantages of the system and method for determining and controlling contamination will be apparent from the following more particular description of preferred embodiments of the system and method as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are described with reference to the following drawings, wherein:

FIG. 1 is a graphical representation of contamination coefficient versus molecular weight;

FIG. 2 is a graphical representation illustrating a comparison of a preferred embodiment of the system for determining contamination with respect to sample mass in a trap and sampling time in accordance with the present invention and the prior art;

FIG. 3 is a graphical representation illustrating analyzed spectral comparisons of the system and method of determining contamination in accordance with a preferred embodiment of the present invention and the prior art;

FIG. 4 is a graphical representation illustrating surface coverage as a function of contamination level in accordance with a preferred embodiment of the present invention;

FIG. 5 is a preferred embodiment of a system of determining contamination in accordance with the present invention;

FIG. 6 is a preferred embodiment of a refractory trap system in accordance with the present invention;

FIG. 7 shows a flow chart of a method of detecting contamination in accordance with a preferred embodiment of the present invention;

FIG. 8 is a diagram illustrating a preferred embodiment of a filtering system in accordance with the present invention;

FIGS. 9A and 9B illustrate a schematic block diagram of a filter device having a bed showing the retention of different species in the bed and a graphical representation of the efficiency of the filter bed with respect to time by measuring the different species, respectively, in accordance with a preferred embodiment of the present invention;

FIG. 10 is a flowchart of a method for monitoring the performance of a gas phase filter system in accordance with a preferred embodiment of the present invention;

FIG. 11 is a schematic diagram of a system that includes a filter system in accordance with a preferred embodiment of the present invention;

FIGS. 12A 12C are graphical illustrations of chromatograms of a gas sample including an average ion scan of the spectra end (FIG. 12C) in accordance with a preferred embodiment of the present invention;

FIGS. 13A and 13B are chromatograms of a second gas sample in accordance with a preferred embodiment of the present invention;

FIG. 14 is a graphical illustration of a chromatogram of a sample of oil free air sampled at a location prior to a filter in accordance with a preferred embodiment of the present invention;

FIG. 15 is a graphical illustration of a chromatogram of a sample of oil free air sampled at a location after the filter in accordance with a preferred embodiment of the present invention;

FIG. 16 is a graphical illustration of a chromatogram of a sample of nitrogen gas sampled at a location prior to a filter bed in accordance with a preferred embodiment of the present invention;

FIGS. 17A and 17B graphically illustrate a chromatogram of a sample of nitrogen gas sampled after the filter system and an average ion scan of the end of the spectra, respectively, in accordance with a preferred embodiment of the present invention;

FIG. 18 graphically illustrates a chromatogram of a empty sampling tube in accordance with a preferred embodiment of the present invention;

FIG. 19 is a flow chart of a method for on-line, real-time monitoring of the performance of a filter system in accordance with a preferred embodiment of the present invention;

FIG. 20 illustrates a schematic block diagram of a system using a system for determining and monitoring contaminants and performance of a filter system in accordance with a preferred embodiment of the present invention;

FIG. 21 illustrates a schematic diagram of system modules in accordance with a preferred embodiment of the system for determining and monitoring contaminants and the performance of a filter system of the present invention;

FIG. 22 illustrates a schematic diagram of a module having a plurality of filter traps of the system shown in FIG. 20 in accordance with a preferred embodiment of the present invention;

FIG. 23 illustrates an alternate view of the module having a plurality of filter traps as shown in FIG. 21;

FIG. 24 illustrates a detailed view of the module having a plurality of filter traps as shown in FIG. 21 along with the plumbing in the manifolds in accordance with a preferred embodiment of the present invention;

FIGS. 25A 25C illustrate schematic diagrams of a device that functions as a concentrator in a filter system in accordance with a preferred embodiment of the present invention;

FIGS. 26A and 26B illustrate schematic block diagrams of a detection system that emulates and detects a deposition process on optical elements in accordance with a preferred embodiment of the present invention;

FIG. 27 is a schematic illustration of a passive sampling system in accordance with various embodiments of the present invention;

FIG. 28 is a flow chart of methods for passive monitoring of contaminants in a semiconductor processing system in accordance with various embodiments of the present invention;

FIGS. 29A 29B are schematic diagrams illustrating one embodiment of a system for detecting airstream backflow in a semiconductor processing system in accordance with a preferred embodiment of the present invention;

FIGS. 30A 30E illustrate schematic diagrams of a device that functions as a concentrator in a filter system in accordance with a preferred embodiment of the present invention; and

FIGS. 31A 31E illustrate a schematic diagram of a system using a device for monitoring contaminants and performance of a filter system in accordance with a various embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a system and method for determining and controlling contamination. Preferred embodiments of the present invention address gaseous contamination as well as the contaminants adsorbed on surfaces, for example, an optical surface. The latter is more critical to the performance of the optical elements.

Table 1 illustrates various species in a cleanroom environment, such as, for example, a fabrication environment using photolithography systems. The low molecular weight species, such as acetone, isopropyl alcohol and low molecular weight siloxanes are the most prevalent in manufacturing environments. Compounds that are most likely to reduce the performance of optics are compounds having a high contamination coefficient or a high molecular weight examples can include, but are not limited to, methoxytrimethyl silane, trimethyl silane and trimethyl silanol. These compounds appear in italics in Table 1 have a higher molecular weight, higher contamination coefficient and an inorganic component. Compounds that negatively impact optical systems may also be described and include refractory compounds such as silanes, siloxanes and iodates, in particular hexamethyldisiloxane (C6-siloxane).

TABLE-US-00001 TABLE 1 Typical concentration, Compound (in cleanrooms) ppbV Isopropyl Alcohol 610.0 Acetone 330.0 Ethanol 134.0 Silane, Methoxytrimethyl- 35.0 Heptane, Hexadecafluoro- 28.0 2-Pentanone 17.0 2-Butanone(MEK) 9.8 Hexane, Tetradecafluoro- 8.9 Butanoic Acid, Heptafluoro- 5.2 Tetrahydrofuran 3.3 3-Buten-2-one 2.5 4-Methyl-2-pentanone(MIBK) 1.9 Silane, Trimethyl(1-Methylethoxy)- 1.7 n-Pentane 1.4 Silanol, Trimethyl- 1.4

Optics design also affects the relative sensitivity of the system to contamination. For example, light transmission is important in transmissive optical systems, like windshields, wherein reflectance approaches zero. High reflectivity systems, where transmission approaches zero, are inherently twice as contamination sensitive as transmissive optical systems because photons pass through any contaminating film twice, whereas light energy is only absorbed or scattered once in transmissive systems.

Describing the effect of molecular films on optical surface properties in terms of mathematics yields equation 1, for reflectance, and equation 2 for transmission. .rho.x(.lamda.)=.rho.(.lamda.)exp[-2.alpha.c(.lamda.)x] Equation 1 .tau.x(.lamda.)=.tau.(.lamda.)exp[-.alpha.c(.lamda.)x] Equation 2 Where: .rho.=reflectance .alpha.=absorbance .tau.=transmittance .lamda.=wavelength .alpha.c=absorbance of a contaminating film, empirically determined

Both transmitted and reflected energy, which is information used in lithography instruments and tools in semiconductor fabrication systems, drop exponentially with the accumulation of molecular films on optical surfaces. In lithography processes, the first order effect of molecular films on lenses is typically a reduction in light intensity due to energy absorbance by the contaminating film. These transmission losses reduce the number of wafers processed per hour, and consequently reduce productivity. This is analogous to the power reductions in spacecraft solar arrays, caused by accumulating molecular films. Secondary effects, in lithography processes, involve a reduction in image uniformity, which reduces critical dimension uniformity and yield.

Photochemical decomposition reactions occur when high-energy photons interact with organic vapors. These reactions form extremely reactive free radicals from otherwise neutral and relatively inert organic molecules. Irrespective of where radical formation occurs, in the gas phase or on the surface of optical elements, the resulting free radicals may react to form much larger organic compounds, which can contaminate optical elements. In severe cases, a polymer layer may be formed on the optical surface. The relationship between the chemical nature of the organic species and wavelength of light it absorbs can affect the nature and severity of optics contamination. For example, I-line or 365 nm wavelength light is energetic enough to break down only a few iodated components, which are not commonly found in clean room air. 248 nm wavelength light, typically used in deep ultraviolet (DUV) lithography for fabricating 250 to 150 nm linewidth devices, is more efficient and reacts with most halogenated organics and may even interact with some common hydrocarbons. 193 nm light, required for less than 130 nm geometries, reacts very efficiently with a wide range of airborne or gaseous molecular organic contaminants. 157 nm optical elements are even more sensitive to environmental conditions than 193 nm optics because this wavelength of light is efficiently absorbed or interacts with nearly all organic species plus oxygen and atmospheric moisture, requiring the exposure area, the area between the final optical element and the wafer, commonly called the free working area, to be purged with an inert, clean, dry, oxygen-free gas.

As the wavelength of light used in the lithography exposure tool decreases, the energy per unit photon increases. These progressively higher energy photons stand a better chance of breaking the bonds of a number of commonly present molecular species, ultimately rendering them into reactive species that stick to optical surfaces. The overall structure of a molecule plays a significant role in the ability of a photon to break any specific bond. Table 2 summarizes optics contamination as the lower wavelengths of electromagnetic spectrum are used to provide for the fabrication of smaller features.

Atmospheric pressure, low K1 factor optical lithography for less than 150 nm critical dimension on 300 mm wafer substrate device production may be the basis of advanced Integrated Circuit (IC) production in the near term. In these technology nodes, lithography-induced critical dimension variations have a particularly acute affect on device characteristics. For example, the standard deviation of propagation delay times for CMOS based ring-oscillators increases from 1% for 300 nm devices to 20% in 250 nm devices. Variations in gate oxide, impurity, and gate lengths were the primary causes of variations in device delay times. Below 200 nm gate length, however, the impact of gate length variation accounts for a remarkable 80% of the effect. The criticality of dimension variation in 150 nm lithography, for example, has lead to a critical dimension control budget of 15 mn, post-etch, 3 sigma. Since exposure dose and image resolution are compromised by optics contamination in proportion to the location and thickness of the contaminating film, contamination needs to be prevented before it occurs.

TABLE-US-00002 TABLE 2 Issue .lamda. = 248 nm .lamda. = 193 nm .lamda. = 157 nm Comments Propensity to Low Moderate Nearly certain Assumes organic form vapor photodeposits in concentrations in nitrogen (<10 the low ppb ppb O2) range Ability to Low Moderate High Based on oxygen photoclean optics absorption surfaces in-situ coefficients and using active organic layer oxygen absorbance Interactions with Aromatics only, Aromatics absorb Nearly all Interaction hydrocarbons moderate very strongly, hydrocarbons determines absorbance other weakly absorb allowable levels of contamination before lens performance suffers

Existing methods of contamination control in lithography involves the use of activated carbon filters and/or some combination of adsorptive and chemisorptive media to adsorb or chemisorb the contaminants in air and gas streams that come in contact with the lens surfaces. In some cases, periodic regeneration of the adsorptive beds by thermal desorption occurs. Passive adsorption is unable to practically capture and retain the lighter hydrocarbons, oxygen, and water that interfere with imaging using 193 nm and 157 nm light. The propensity to form photodeposits, ability to photoclean, and interaction of hydrocarbons is tabulated relative to different wavelengths of light in Table 2.

Filter systems for contamination control are described in U.S. application Ser. No.: 10/205,703, filed on Jul. 26, 2002 entitled "Filters Employing Porous Strongly Acidic Polymers and Physical Adsorption Media", U.S. application Ser. No.: 09/969,116, filed on Oct. 1, 2001 entitled "Protection of Semiconductor Fabrication and Similar Sensitive Processes", and U.S. application Ser. No. 09/783,232, filed on Feb. 14, 2001 entitled "Detection of Base Contaminants In Gas Samples", the entire teachings of the above referenced applications are being incorporated herein by reference in their entirety.

FIG. 1 is a graphical representation 20 of contamination coefficient 22 versus a molecular weight 24. Note that a higher contamination coefficient means that it is more likely to contaminate system optics. The nearer term 193 nm wavelengths show some correlation between the contaminants molecular weight and its ability to contaminate the lens. Consequently, while the higher molecular weight species are of greater immediate concern for lens contamination, the lower boiling point materials, which are typically in higher concentration in semiconductor cleanrooms as shown in Table 1, can become a concern due to their much higher concentration and ability to adsorb photon energy at progressively shorter wavelengths. Moreover, particularly at 157 nm, oxygen and water need to be removed from the light path because they also absorb photon energy.

Existing systems have many disadvantages including passive adsorption systems that do not effectively remove low molecular weight organic materials; the removal efficiency and capacity of passive adsorption systems are proportional to the concentration of the impurities. In this application, the inlet concentrations are very low, making efficiency and capacity correspondingly low; and on-site regeneration of passive adsorption beds requires periodic temperature increases to regenerate the beds. Since most advanced lithography systems must maintain air and gas temperature stability at typically less than 100 milliKelvin, to avoid heating or cooling the optics, which change their optical characteristics, this strategy is impractical in advanced lithography.

FIG. 2 is a graphical representation 30 illustrating a comparison of a preferred embodiment of the system for determining contamination with respect to sample mass in a collection device or contamination trap and sampling time in accordance with the present invention and the prior art. An extended duration sample time, sample time 40, is used wherein the gas sample volume is not limited by the low molecular weight breakthrough volume, as is the case with the prior art method using sample time 38. In a preferred embodiment, the sampling time is at least six hours long and is preferably in a range of six hours to twenty-four hours. Higher capacity traps yielding longer collection times may be necessary for certain applications.

The extended time sampling method in accordance with a preferred embodiment of the present invention, collects higher masses of higher molecular weight compounds, which contribute to the contamination in the gas supply and which reduce the performance of optical elements more so than lower molecular weight compounds. Both high and low molecular weight compounds contribute to the contamination level but are operative at different rates. The high molecular weight compounds contribute to first order contaminating effects as they cause more damage to the optical systems even if present at low concentrations than low molecular weight compounds which contribute to third and fourth order effects. The collection device in accordance with a preferred embodiment is driven beyond saturation or breakthrough capacity to quantitatively measure the equilibrium concentration of low molecular weight compounds. The breakthrough volume is the amount of gas sample volume required to go beyond the absorbent capacity of the collection device. It should be noted that contaminates may be inorganic materials which may be carried by organics to the optical element. This extended time sampling method can also use different types of sample collecting media including those with adsorptive properties close to that of the optical surfaces of interest.

A preferred embodiment of the present invention includes "glass" or "coated glass" based adsorptive contamination traps. These contamination traps have not been used in the past due to their limited ability to collect and retain lower molecular weight species. These materials have surface properties identical or similar to properties of the optical elements used in the optical systems of photolithography tools. Other materials that emulate the surface properties of these optical elements that generate contamination can also be used.

In a preferred embodiment, the extended time sampling method may be extended from a few hours to several days and even weeks. The amounts of analyte collected represents the average value over time for compounds that have not reached their breakthrough time as illustrated by line 36 at sample time 2, line 40, and an average equilibrium concentration for those species that have already reached their breakthrough volume as illustrated by line 34 at sample time 2, line 40.

With respect to higher molecular weight species, the internal surface of the sampling lines and/or manifolds are kept at equilibrium with the gas phase sample, and therefore do not interfere with the sample collection process. In a preferred embodiment, between sampling sessions, flow through the sampling lines and/or manifolds is maintained.

FIG. 3 is a graphical representation 50 illustrating spectral analysis comparisons of the system and method of determining contamination in accordance with a preferred embodiment of the present invention and the prior art. The extended time sampling method of the present invention offers better sensitivity for components having high boiling points as illustrated by lines 52, 56. The results of the extended time sampling method in accordance with a preferred embodiment of the present invention better represent contamination on the optical surface, given the improved high molecular weight sample collection method of the present invention. A preferred embodiment of the system of the present invention provides the ability to use the actual optical surface of interest as the collection medium which in turn allows alignment of sampling surface properties and optical surface properties thereby making the analysis results more meaningful to the prediction of optics contamination.

The extended time sampling method in accordance with a preferred embodiment may reduce and preferably eliminate the uncertainties of sample loss on sample lines and/or manifolds. The extended time sampling method's simplicity minimizes the effect of uncontrolled contamination by personnel deploying the traps. Consequently, less training and experience are required to collect samples.

FIG. 4 illustrates graphically surface coverage as a function of contamination level showing greater surface mass coverage per unit concentration in accordance with a preferred embodiment of the present invention. FIG. 4 illustrates this relationship for higher molecular weight components at the upper left with the lower molecular components towards the lower right of the graph. For a given concentration, the higher molecular weight compounds collect on surfaces more readily than do lower molecular weight species. One of the problems with the prior art method is that due to the shorter sampling times, much of what little sample is available for collection collects on the sample tube walls and manifold surfaces, all upstream of the collection trap, and never reaches the trap. This phenomenon causes a further loss of high molecular weight sample mass. Moreover, heated sampling lines and/or manifolds, which could ameliorate the problem, are not practical in the production cleanroom environment.

FIG. 5 is a diagram of a preferred embodiment of a system 100 for determining contamination in accordance with the present invention. The preferred embodiment of the apparatus includes a tubular collection device 102 having an inlet port 104 and an outlet port 106. In a preferred embodiment, the collection device includes, absorptive materials 108 such as, for example, glass spheres of a given size. In a preferred embodiment, crushed glass spheres are used. In another preferred embodiment, the absorptive material 108 is the polymer Tenax.RTM. supplied by, for example, Supelco. Tenax.RTM. has a high capacity for high boiling point compounds and operating Tenax.RTM. past low molecular weight breakthrough capacity allows the capture of a meaningful and analyzable mass of high molecular weight compounds. To collect a sample, an end cap in the inlet post is removed, allowing gas from a gas source to pass through the inlet port 104. Laser light may be directed through the sampling tube in a preferred embodiment of the present invention. The free radicals of the contaminants present in the gas sample may bond with the absorptive media 108 in the collection device 102.

In a preferred embodiment of the system for controlling contamination, multiple sample tubes and blank collection devices may be used. The collection device or refractory trap is applicable to both high pressure sampling, for example, purge gas, venting to the atmosphere assuming sufficient pressure and filter sampling, wherein the traps are connected to a vacuum source. The flow is controlled by an easily changeable critical orifice.

In a preferred embodiment, the trap contains three sample tubes, one blank and two active sample devices. Chemical analysis of the data may be correlated to transmission or image uniformity loss of the lithography tool, for example, using a regression analysis which weights first, second, third and fourth order effects: Uniformity or Intensity=a [C.sub.6-siloxane]+b[C.sub.6 C.sub.30]+c[C.sub.3 C.sub.6]+d[C.sub.1 C.sub.5] herein the parenthetic expressions are indicative of the concentration of species. First and second order contaminating effects have a greater impact on contamination of optical systems than third or fourth order contaminants and typically show a greater contamination coefficient (e.g. a>b>c>d). The first order contaminants may comprise high molecular weight refractory organics such as, for example, C.sub.6 siloxanes and C.sub.6 iodide with an inorganic component which is not volatilized through combination with oxygen. Second order contaminants may comprise high molecular weight organics, such as, for example, compounds including carbon atoms within the range of approximately six to thirty carbon atoms (C.sub.6 C.sub.30). Third order effects can arise due to the contaminating effects of organics such as C.sub.3 C.sub.6 that have approximately three to six carbon atoms. Further, fourth order contaminants Include organics such as, for example, methane, that have approximately one to five carbon atoms.

In preferred embodiments of the system in accordance with the present invention, a refractory trap may be used both upstream and downstream of any in-line filtration system. FIG. 6 is a preferred embodiment of a refractory trap system 120 in accordance with the present invention. As described herein before refractory compounds include at least siloxanes such as, for example, hexamethyldisiloxane (C.sub.6), silanes such as, for example, C.sub.3-silane, silanols such as, for example, C.sub.3 and iodates. The refractory trap system 120 includes a conduit 121 in communication with a gas source and through which a gas sample is carried with pressures ranging between approximately 1 to 120 psi. The gas sample is carried downstream to a pressure cavity 122. A pressure relief valve 123 allows the continuous flow of gas to ensure that the pressure cavity walls are in equilibrium with the gas phase of the gas sample. The refractory trap system 120 includes active sampling traps or collection devices 124 and a blank trap 125 in the trap cavity 126. The active sampling trap elements 124 may include an absorptive medium such as, for example, the polyler Tenax.RTM.. The gas sample flow in active elements is approximately 0.11 lpm. The blank trap 125 is not in communication with the gas source or pressure cavity and as such is not removing any contaminants. The outflow gas stream from the active collection devices 124 flows downstream into a manifold 127 which is in fluid communication with a vacuum line 130, via an orifice 129. A pressure/vacuum regulator valve 108 is disposed between the manifold and the orifice 129 to regulate pressure. The refractory trap system 120 provides for both a low pressure application or a high pressure application using a single design.

In a preferred embodiment, the gas supply may include a particular constituent such as hydrogen gas which may be used to clean the surfaces of the collection devices or, surfaces of optical systems that have been contaminated by a surface contaminant, for example, SiX. The gas additive combines with the surface contaminant to form a volatile compound that is then purged from the system. For example, SiX combines with hydrogen gas to form silane (SiH.sub.4) which is volatile and is purged. The purge gas, is preferably in the ultra high purity gas level allowing the collection device to be placed upstream and downstream of typical in-line filters.

A sample report derived from a collection device may comprise the following information: Contact information: Name, address, phone, email of person sending the sample Tool #: Gas sampled: N2 Air Sample location: Upstream of filter Downstream of filter Interstack Sample start date: Sample end date: Date received: Report date: Upstream Sample: C2 C5: X ppb* (*equilibrium concentration) C6 C30: Y ppb Total siloxanes: z ppb Total sulfur compounds: Past history on this sample location:

In another preferred embodiment the collection device is located directly in contact with the airstream, thereby avoiding sample line contamination and using either passive diffusion or an active flow to collect the sample.

FIG. 7 is a flow chart of the method 150 of detecting and removing contamination in accordance with a preferred embodiment of the present invention. The method includes the step 152 of delivering a gas sample to a collection device. In a preferred embodiment, the collection device is as described with respect to FIG. 5 and/or FIG. 6. The method further includes the step 154 of absorbing contaminants contained in the gas sample in the collection d