KR20040098054A - Fluid assisted cryogenic cleaning - Google Patents

Abstract

The present invention is directed to fluid assisted low temperature cleaning of substrate surfaces requiring precise cleaning such as semiconductors, metals and dielectric films. The method includes applying a fluid selected from the group consisting of a high vapor pressure liquid, a reactive gas, and a vapor of the reactive liquid onto the substrate surface to clean the substrate surface by cold cleaning to remove contaminants thereafter.

Description

FLUID ASSISTED CRYOGENIC CLEANING

Processes for cleaning or surface fabricating silicon wafers in the presence or absence of various layers of film are important for integrated circuit fabrication methods. Removal of particles and contaminants from the wafer surface is performed in several critical process steps during the fabrication of integrated circuits. At a 0.18 μm technology node, 80 of 400 steps or 20% of the fabrication sequence is allocated for cleaning. The challenge of the cleaning technique is increased with various types of contaminants to be removed in the film, form and shear line (FEOL) and trailing line (BEOL) cleaning methods. Removal of particles is an important part of this cleaning.

For fabrication without the integration of integrated circuits, the International Technology Roadmap for Semiconductors (ITRS) suggests that the critical particle size is half the DRAM (half pitch) (Ref. [1]). Thus, at a 130 nm technology node with a DRAM 1/2 pitch of 130 nm, the critical particle size is 65 nm. Therefore, particles larger than 65 nm must be removed to produce a defect free device.

For smaller particles, these small particles are difficult to remove because the ratio of adhesion to removal force increases. For ultrafine particles, the main force that bonds the particles to the surface is the van der Waals forces. This force depends on the size of the particle, the distance of the particle from the substrate surface and the Hamaker constant. The van der Waals forces for spherical particulates on a flat substrate are given by Equation I:

Where

A ₁₃₂ is the Hamaker constant of a system consisting of particles, surfaces and mediation media;

d _p is the particle diameter;

Z ₀ is the distance of the particles from the surface.

The Hamaker constant A ₁₃₂ for the composite system is given by the following equation II:

The relationship of the _hamaker constants of two different materials is expressed as the geometric mean of the individual _hamaker constants as A _ij = (A _ii x A _jj ) ^1/2 , where A _ii and A _jj are the _hamaker constants of materials i and j to be. This is theoretically calculated using the Lifshitz or London model. Hamaker constants for particles and surfaces used in integrated circuit fabrication methods are provided in references [2] and [3], which are smaller when the mediation medium is a liquid comparable to air. Thus, van der Waals forces, which are directly proportional to the Hamaker constant, decrease when there is a liquid layer between the particles and the surface.

In addition to the difficulty of removing small particles from the surface, there are several types of organic and metal-organic contaminants to be removed. The need for better switch speed and circuit performance has resulted in new dielectric materials (dielectric constants less than 3) and metals that reduce the RC delay constant in the circuit. When the metal of choice was copper, some challenges were added to the process integration scheme. For aluminum connections, metal patterning was performed by dielectric deposition followed by reactive ion etching (RIE) of aluminum. Using copper, a dielectric film was first deposited and etched to form vias and trenches, and then copper was deposited within this etched feature. Subsequently, excess copper was removed using chemical mechanical polishing (CMP) to planarize the surface for subsequent layers of film. Such a method of forming a copper connection to a BEOL is known as the Dual Damascene method.

After forming vias and trenches by dielectric etching, a large amount of fluorinated polymer residue is left both on the surface of the wafer and inside the features shown in FIG. These residues occur for the sidewall passivation during the etching process, partly during the anisotropic etching. The etch residue should be cleaned prior to the deposition of successive film layers, ie copper wall Ta / TaN films, copper seed layers and finally electrochemical filling of features with copper in the damascene process.

The dimensions of the features used for connection in BEOL are typically about 0.13 μm. For cold cleaning that effectively performs removal of sidewall residues from within the features, as shown in FIG. 1, the cold particles must be less than 0.13 μm in size. In addition, these particles must reach the surface of the wafer at a rate sufficient to impart the momentum transfer needed to move the sidewall residue.

There are three mechanisms by which surface cleaning is performed: (1) transfer of momentum by low temperature particles to overcome adhesion of slurry particles to the wafer surface, (2) traction of cleaning gas to remove particles moved to the wafer surface, and ( 3) Dissolution of organic contaminants by liquid formed at the interface of low temperature particles and wafer surface.

In CO ₂ low temperature cleaning, gas flow on the wafer surface forms a boundary layer. The C0 ₂ cold particles must travel through the boundary layer to reach the wafer surface and the contaminant particles to be removed. During the flight through the boundary layer, the speed is reduced due to the traction by gaseous CO ₂ in the boundary layer. When the thickness of the boundary layer is h, snow particles must be introduced into a conventional component layer having a speed of h / t or more, where t is the time it takes to cross the boundary layer and reach the wafer surface. The relaxation time of the particles across the boundary layer is given by the following equation:

Where

a is the particle radius;

ρ _p is the particle density;

η is the viscosity of the gas;

C _c is the Cunningham slip correction coefficient given by Equation 2:

C _c = 1+ 1.246 (λ / a) + 0.42 (λ / a) exp [-0.87 (a / λ)]

Where

λ is the mean free path of the gas molecules.

Since the CO ₂ cold wash is performed at atmospheric pressure, the Kuningham slip correction factor in Equation 1 is 1 when the cold particle size is greater than 0.1 μm.

Thus, for CO ₂ snow particles that have sufficient momentum to remove foreign material from the wafer surface and inside the features, the time across the boundary layer should be less than the relaxation time, in which case they exceed 36% of the initial rate. To reach the surface. Equation 1 shows that relaxation time decreases with particle size. Thus, smaller size particles do not reach the wafer surface at a rate sufficient to effectively clean the inner walls of the ultrafine vias and trenches.

In general, the prior art methods use CO ₂ or argon cold spray to remove foreign material from the surface. See, for example, U. S. Patent No. 5,931, 721 (inventive name: aerosol surface process); US Patent No. 6,036,581 (name of the invention: substrate cleaning method and apparatus); U.S. Patent No. 5,853,962 (name of the invention: photoresist and redeposition removal using carbon dioxide jet spray); U.S. Patent No. 6,203,406 (inventive name: Aerosol Surface Finish); And US Pat. No. 5,775,127 (name of the invention: highly disperse carbon dioxide snow apparatus). In all of the above prior art patents, foreign material is removed from the relatively planar surface by the physical forces involved in the transfer of momentum to the contaminants. Prior art methods are inefficient in removing small particles of less than 0.3 μm because the adhesion between contaminant particles and the substrate is strong. This cleaning method is also unsuitable for features with high aspect ratios such as vias and trenches in the fabrication of backend line integrated devices that require the removal of small ultrafine particles and composite polymer residues generated by dielectric etching methods.

U.S. Patent No. 6,332,470 (named an aerosol substrate cleaner) discloses the use of steam alone or a combination of steam and high pressure liquid droplets for cleaning semiconductor substrates. Unfortunately, liquid impacts do not have sufficient momentum transfer capacity as solid CO ₂ and are therefore not effective at removing smaller particles. U.S. Patent No. 5,908,510, entitled Invention Removal Residue by Supercritical Fluid, discloses the use of supercritical fluid or liquid C0 ₂ with low temperature aerosols. Since CO ₂ is a non-polar molecule, the solubility of polar foreign substances is significantly reduced. In addition, the device is expensive because liquid or supercritical CO ₂ formation requires high pressure (greater than 75 psi for liquid and 1080 psi for supercritical). U. S. Patent No. 6,231, 775 proposes the use of sulfur trioxide gas alone or in combination with other gases to remove organic material from a substrate as in ashing. This vapor phase cleaning is inadequate to remove the crosslinked photoresist formed during etching in a typical dual damascene integration scheme using low k materials such as carbon doped oxides.

As such, crosslinking and bulk photoresists, post-etch residues and ultrafine sizes from semiconductor wafers, metal films and other substrates requiring fine cleaning as well as particles, foreign materials and chemical residues, and from inside high aspect ratio features There remains a need for effective and efficient removal of contaminants, including homogeneous or heterogeneous contaminants consisting of particles of.

Summary of the Invention

The present invention provides new and improved methods for cleaning substrate surfaces that require precise cleaning, such as semiconductors, metals and dielectric films.

The present invention includes a cleaning method that removes contaminants from a substrate surface requiring precise cleaning. This method is used prior to or concurrently with the cold cleaning to remove foreign substances and contaminants from the substrate surface. This method is applied to fluids selected from high vapor pressure liquids, reactive gases or vapors of reactive liquids, depending on the contaminants to be removed from the substrate surface. The fluid is preferably in contact with the surface for up to 20 minutes. This creates an environment that removes contaminants from the surface or reduces adhesion to the surface and can subsequently be removed using cold cleaning.

The present invention relates to the use of a liquid or vapor cleaning method performed concurrently with or prior to low temperature cleaning to assist in the removal of foreign matter and contaminants from semiconductor surfaces and other surfaces involved in fine cleaning.

Aspects of the invention are described with reference to the drawings:

1 shows cleaning of post-trench etch residues in a dual-damacin structure. Left image is SEM of post-trench etch structure with etch residue present. The image on the right is an SEM of post-trench etch structure after the sequence of plasma and wet clean steps.

FIG. 2 is a graph showing the efficiency of particle removal comparing particle size for both standard low temperature cleaning and the present liquid assisted cleaning process.

3 shows a schematic of a conventional CO ₂ low temperature cleaning system.

Liquid Assisted Cleaning Methods and Examples

The liquid used in this method is a high vapor pressure liquid that reduces the van der Waals forces between the foreign material and the substrate surface, such as the semiconductor wafer surface or the film surface. The high vapor pressure liquid is sprayed onto the surface of the substrate. Initial spraying of the liquid reduces subsequent van der Waals forces and thus allows for subsequent low temperature cleaning to more easily remove foreign material from the substrate surface. If the upstream process before the cold wash is an aqueous process, the liquid may also remove a large amount of water prior to the cold wash, as in co-pending US patent application Ser. No. 10 / 215,859. In addition, the high vapor pressure liquid can act to dissolve organic contaminants from the surface. The particular high vapor pressure liquid is selected depending on the organic contaminants contained on the substrate surface. Those skilled in the art are interested in the type of liquids that dissolve conventional organic contaminants.

High vapor pressure liquids suitable for use in the present invention include ethanol, acetone, ethanol-acetone mixtures, isopropyl alcohol, methanol, methyl formate, methyl iodide, ethyl bromide, acetonitrile, ethyl chloride, pyrrolidine and tetrahydro Including but not limited to furan. However, any liquid having a high vapor pressure can be used. The high vapor pressure liquid is easily evaporated off the surface of the substrate without having to be dried by heating or rotating the substrate. The liquid also preferably has a low freezing point and is polar in nature. The low freezing point of the liquid ensures that any remaining liquid left on the wafer surface during cold cleaning will not freeze due to the drop in wafer temperature that can be obtained during the cold cleaning process. The polarity of the liquid assists in the dissolution of organic and inorganic contaminants on the wafer surface. Preferably, the vapor pressure of the liquid is above 5 kPa at 25 ° C., the freezing point of the liquid is below −50 ° C. and the dipole moment is above 1.5D.

High vapor pressure liquids can be used for any substrate surface that requires precise cleaning, but preferred surfaces include metal and dielectric films as well as semiconductor surfaces. Thus, when the term "semiconductor", "metal film", "dielectric film" or "wafer" is used herein, it is intended that the same process can be applied to other substrate surfaces. Other surfaces include hard disk media, optics, GaAs substrates, and films in compound semiconductor fabrication methods. The examples provided herein are not intended to limit the invention.

In one embodiment of the invention, the high vapor pressure liquid is sprayed onto the surface of the semiconductor wafer at a temperature of 30 to 50 ° C. The liquid can be sprayed into a thick film or thin layer. Preferably the layer is at least 5-10 mm thick. Preferably, it is sprayed using a Teflon mist nozzle used in a wet bench to spray deionized water onto the wafer surface. However, any other nozzle used in the art may be used. The wafer is preferably coated with liquid for at least 1 minute, preferably at most 10 minutes. The liquid may be applied to the surface at one time during this time, or may be sprayed several times to keep the wafer surface wet. In addition, the wafer can be rotated at about 100 rpm while the liquid is sprayed to ensure a uniform coating of the wafer surface.

Following this wetting period, low temperature spraying is started. Cold spray methods may use carbon dioxide, argon or other gases and are well known in the art. Any known technique can be used and examples of CO ₂ low temperature cleaning are described below. The result of the initial application of high vapor pressure liquids is to reduce the Hamaker constant and thus van der Waals forces. This application lowers the adhesion of contaminants to the wafer surface and removes contaminants from the wafer surface more easily than using low temperature cleaning alone.

Alternatively, the liquid can be applied simultaneously with the cold wash. In this case, for example, a second nozzle for liquid spraying can be mounted with the first nozzle used for the CO ₂ low temperature cleaning. The liquid can preferably be applied to a thin layer and the CO ₂ low temperature cleaning continues simultaneously with the spraying of the liquid on the substrate.

As a result of the use of high vapor pressure liquids, the removal of particulate contaminants by low temperature cleaning is significantly improved. 2 shows the efficiency of particle removal comparing particle size for standard cold cleaning as well as liquid assisted cold cleaning. Removal of particles having a size of less than 0.76 μm is significantly improved with the use of this liquid assisted CO ₂ low temperature cleaning process over standard CO ₂ low temperature cleaning. For particle sizes ranging from 0.98 μm to 2.50 μm, there is not much difference in particle removal effect between the present liquid assisted low temperature cleaning and the use of a standard CO ₂ low temperature cleaning process.

Steam assisted cleaning and examples

The reactive vapor of the reactive gas or liquid can be used to assist in the removal of contaminants. The reactive gas or vapor is selected according to the reactivity of the contaminants on the substrate surface. Generally, reactive gases or vapors are used to remove organic photoresist and fluoropolymer etch residues within features on the substrate surface. After reacting with the contaminants, the gas / vapors preferably produce by-products in gaseous form. (Hereafter, reference to the reactive gas may include a reactive vapor of a liquid, and reference to the reactive vapor may include a reactive gas to facilitate the technique of the present invention.)

In semiconductor wafer cleaning methods, the contaminants to be removed include not only particle contaminants but also films of organic, inorganic and metal-organic residues at various stages of microelectronics fabrication in both the FEOL (shear line) and BEOL methods. These films cannot be removed by purely physical mechanisms. Chemical assistance for any physical mechanism of removal is necessary to meet the cleanliness requirements. In the present invention, gas phase cleaning is the chemical means of cleaning, while low temperature cleaning is primarily the physical mechanism of cleaning. Two processes operating in series or sequentially with each other can completely remove homogeneous or heterogeneous contaminants.

Examples of reactive vapors that may be used in the process may be vapors of high vapor pressure liquids, including but not limited to acetone, ethanol-acetone mixtures, isopropyl alcohol, methanol, methyl formate, methyl iodide and ethyl bromide Do not. It may also include ozone, water vapor, hydrogen, nitrogen, nitrogen oxides, nitrogen trifluoride, helium, argon, neon, sulfur trioxide, oxygen, fluorine, or a fluorocarbon gas or combination of gases. The gas or vapor should be reactive with the fluoropolymer etch residue as well as the organic photoresist inside the feature. It is also preferred that the reaction byproduct is a gas so that it can be removed from the cleaning chamber by the flow of nitrogen gas. Preferred gases and liquid vapors include isopropyl alcohol, ethanol-acetone mixtures, methanol, ozone, water vapor, nitrogen trifluoride, sulfur trifluoride, oxygen, fluorine and fluorocarbon gases.

In post-etch cleaning applications, cold particles cannot be obtained inside the high aspect ratio features of vias and trenches. Gas or vapor needs to diffuse into these features efficiently. The gas or vapor is then converted into gaseous byproducts that can react chemically with the polymer residue and be removed from the surface by the flow of nitrogen across the substrate surface. Alternatively, it can be introduced into a separation chamber maintained at low pressure. The gas / vapor phase reaction in this chamber can be carried out at temperatures of up to 200 ° C. Following this cleaning process, the wafer can be transferred to the second cleaning chamber at atmospheric pressure at which low temperature cleaning occurs.

During processing, vapor can be condensed on the wafer surface. By proper choice of steam, condensation can also lower the Hamaker constant and thus the adhesion of the particles to the surface. This condensation can assist in particle removal by cold washing.

The gas or vapor can further increase the reactivity with contaminants to be removed using free radical initiators such as ultraviolet light, X-rays, excimer lasers, corona discharges or plasma to generate reactive chemical species. Combined with physical cleaning of snow or low temperature aerosols to remove non-reactive contaminants. Similar cleaning mechanisms are observed in wet cleaning and dual frequency plasma cleaning using downstream MW plasma generating chemical species to react with contaminants and RF plasma generating ion bombardment.

In one embodiment of the present invention in combination with CO ₂ low temperature cleaning, the vapor of the liquid is sprayed through a nozzle attached to the same arm as the CO ₂ low temperature nozzle. The nozzle may be a small stainless steel perforator with a diameter of 1/4 to 1/2 ", or a nozzle specifically designed to have corona wires along the axis initiating discharge to the vapor. The nozzle is preferably from about 10 ^o to the substrate surface. an angle of 90 ^o. steam is also sprayed through a shower head located on the substrate surface can be reliably uniformly coating the substrate surface during vapor delivery, the substrate preferably maintains the same temperature as the steam. steam If condensation of is desired, the substrate can be maintained at a temperature below the vapor that initiates condensation of the vapor into a thin film of liquid on the substrate surface, but if the vapor does not have sufficient reactivity to the type of contaminant presented, the vapor is free radicals. It may be reactive with the aid of the initiator The vapor is preferably sprayed onto the substrate surface for up to 20 minutes. It can be sprayed intermittently Preferably, a single type of steam can be used but a mixture of steam can be used simultaneously or sequentially to remove contaminants if desired.

According to the invention the spraying of the reactive gas or vapor can take place in the same chamber as the low temperature cleaning or in a separate chamber. In addition, low temperature cleaning may be initiated simultaneously with or immediately after use of the reactive gas or vapor. Depending on the reactive gas or vapor used, for example water vapor, it may be desirable to sweep the chamber of this vapor prior to the start of the low temperature cleaning.

As a result of the use of reactive gases or vapors, in particular, the removal of contaminants from features etched on the substrate surface is significantly improved. This cleaning method is particularly beneficial for removing homogeneous contaminants such as films of post etch residues on the sidewalls of vias and trenches or photoresists remaining after etching.

Standard CO ₂ Low temperature cleaning

After or at the same time as the fluid cleaning process, a standard low temperature cleaning is carried out. Standard CO ₂ low temperature cleaning processes are described in US Pat. No. 5,853,962, which is incorporated herein by reference. As an example of a typical CO ₂ low temperature cleaning system, see FIG. 3. The cleaning vessel 12 provides an ultra clean sealed or sealed cleaning zone. There is a wafer 1 held on the platen 2 by vacuum in this cleaning zone. The platen with the wafer is maintained at a controlled temperature of 100 ° C. or less. Liquid CO ₂ from the cylinder at room temperature and 850 psi is first passed through a sintered in-line filter 4 to filter very small particles from the liquid stream to make carbon dioxide as pure as possible and to reduce contaminants in the stream. The liquid C0 ₂ is then allowed to expand through small aperture nozzles, preferably 0.05 "to 0.15" in diameter. Rapid expansion of the liquid causes a temperature drop resulting in the formulation of solid CO ₂ snow particles entrained in the gaseous CO ₂ stream flow at a rate of about 1 to 3 volume feet per minute. A stream of solid and gaseous CO ₂ is directed to the wafer surface at an angle of about 30 ^° to about 60 ^° , preferably about 45 ^° . The nozzle is preferably located at a distance of about 0.375 "to 0.5" measured along the line of view of the nozzle to the wafer surface. During the cleaning process, the platen 2 moves back and forth on the track 9 in the y direction, while the arm of the cleaning nozzle moves linearly on the track 10 in the x direction. This results in a rasterized cleaning pattern on the wafer surface that can pre-set the step size and scan speed as needed. The humidity in the cleaning chamber is preferably kept as low as possible, for example, below the -40 ° C dew point. Low humidity prevents condensation and freezing of moisture on the wafer surface from the atmosphere during the cleaning process to form crystalline crosslinks between contaminant particles and the wafer surface, increasing adhesion between them. Low humidity can be maintained with a stream of nitrogen or clean dry air.

It is also important that the electrostatic charge in the cleaning chamber is neutralized throughout the cleaning process. This is done by the bipolar corona ionization rod 5. The system also has a polonium nozzle mounted directly behind a CO ₂ nozzle that enhances charge neutralization of the wafer mounted on the electrically polished platen. Electrostatic charge is assisted by low humidity developed into triboelectricity and maintained in the cleaning chamber due to the flow of CO ₂ across the wafer surface through the nozzle.

For particulate contaminants, the removal mechanism is primarily to overcome the adhesion of contaminant particles on the wafer surface by momentum transfer of CO ₂ cold particles. Once the particles are "loose", the traction of the gaseous CO ₂ is removed from the surface of the wafer. The cleaning mechanism for organic film contaminants is the formation of a thin layer of liquid CO ₂ at the interface and surface of the organic contaminants due to the impact pressure of the low temperature CO ₂ on the wafer surface. The liquid CO ₂ may then dissolve and remove the organic contaminants from the wafer surface.

The aspects and examples herein are intended to illustrate, but not limit, the invention. Other embodiments that can be used in the method are readily apparent to those skilled in the art. Such an aspect shall also fall within the scope of the present invention.

references

[1] International Technology Roadmap for Semiconductors 2001 Edition.

[2] Handbook of Semiconductor Wafer Cleaning Technology Science, Technology and Applications, Edited by Werner Kern, Noyes Publications, 1993.

[3] Particle Control for Semiconductor Manufacturing, Edited by R. P. Donovan, Marcel Dekker Inc., 1990.

Claims (21)

(a) applying at least one fluid selected from the group consisting of a high vapor pressure liquid, a reactive gas and a vapor of a reactive liquid to the substrate surface; And (b) low temperature cleaning of the substrate surface; A method for removing contaminants from the surface of the substrate comprising a fine cleaning, including. The method of claim 1, The process in which steps (a) and (b) are carried out continuously. The method of claim 1, The process in which steps (a) and (b) are performed sequentially. The method of claim 1, At least one fluid consists of ethanol, acetone, ethanol-acetone mixtures, isopropyl alcohol, methanol, methyl formate, methyl iodide, ethyl bromide, acetonitrile, ethyl chloride, pyrrolidine, tetrahydrofuran and mixtures thereof High vapor pressure liquid selected from the group. The method of claim 1, At least one fluid is a vapor of a reactive liquid selected from the group of liquids consisting of ethanol, acetone, ethanol-acetone mixtures, isopropyl alcohol, methanol, methyl formate, methyl iodide, ethyl bromide and mixtures thereof. The method of claim 1, At least one fluid is at least one reactive gas selected from the group consisting of ozone, water vapor, hydrogen, nitrogen, nitrogen oxides, nitrogen trifluoride, helium, argon, neon, sulfur trioxide, oxygen, fluorine, fluorocarbon gas and mixtures thereof . The method of claim 1, At least one fluid is a reactive gas or vapor selected from the group consisting of icepropyl alcohol, ethanol-acetone mixtures, methanol, ozone, water vapor, nitrogen trifluoride, sulfur trioxide, oxygen, fluorine, fluorocarbon gas and mixtures thereof. The method of claim 1, Wherein the fluid maintains contact with the surface for up to 10 minutes prior to commencement of the low temperature cleaning. The method of claim 8, Wherein the fluid maintains contact with the surface for less than two minutes prior to the commencement of the cold wash. The method of claim 1, The size of the contaminants is less than 0.76 μm. The method of claim 1, The size of the contaminants is less than 0.13 μm. The method of claim 1, Wherein the high vapor pressure liquid has a vapor pressure of greater than about 5 kPa and a freezing point of less than about −50 ° C. at 25 ° C. The method of claim 1, Wherein the high vapor pressure liquid has a dipole moment greater than about 1.5D. The method of claim 1, Wherein the high vapor pressure liquid remains on the in-layer surface for at least 5 kPa for less than 10 minutes, preferably less than 2 minutes, prior to commencement of low temperature cleaning. The method of claim 4, wherein Wherein the high vapor pressure liquid comprises an additional step of removing a large amount of water from the substrate surface. The method of claim 1, The substrate surface is a semiconductor, metal or dielectric film. The method of claim 1, One or more fluids react with contaminants on the surface to form volatile gas byproducts; Further comprising maintaining the reactive gas or vapor in contact with the surface for up to 20 minutes, and removing the gaseous by-products prior to commencement of the low temperature cleaning. The method of claim 17, Introducing a reactive gas or vapor into the chamber containing the substrate under low pressure and / or at a temperature of 200 ° C. or less. The method of claim 18, Removal of the by-products comprises sweeping the chamber with nitrogen or clean dry air. The method of claim 17, A method of applying a reactive gas or vapor to a surface in the presence of a free radical initiator to generate reactive chemical byproducts and contaminants from the reactive gas or vapor. The method of claim 20, The free radical initiator is ultraviolet, x-ray, laser, corona discharge or plasma.