Patent Application
20080047579
The present invention has applicability to a process which involves the use of a cleaning agent to remove from a metallic surface a residue that comprises a metallic material that is less reactive with the cleaning agent than the metal comprising the metallic surface. An example of such a process involves removing residue from the surfaces of walls that form a chamber in a reactor in which chemical vapor deposition (CVD) or atomic layer deposition (ALD) is used to form a metallic film on an object, for example, a capacitor. During the use of such processes, the surfaces of the walls tend to become coated with residue which comprises constituents of the metallic film and which, if not removed periodically, causes problems of the type described hereinabove. Removal of the residue is effected by the cleaning agent which causes the residue to volatilize; the resulting vapor is moved away from the surface, for example, out of the chamber of the reactor. As the residue is removed, the cleaning agent is then capable of attacking and damaging the exposed metallic surfaces of the walls comprising the chamber of the reactor.
As mentioned above, the metal comprising the surface to which the residue is adhered is more reactive with the cleaning agent than the metallic material that comprises the residue. Examples of such metals include stainless steel, hastalloy, nickel coated carbon steel, and various alloys of alluminum. In preferred form, the metal is aluminum, preferably an aluminum alloy, for example, aluminum (6061).
The source of the residue which forms on the metallic surface can be any metallic material that is capable of being deposited as a solid film, as it is formed from a precursor gas, on an object and that is capable of reacting with the plasma cleaning agent at a rate of reaction that is slower than the rate at which a metal of the metallic surface reacts with the plasma cleaning agent.
Examples of the metallic material are a transition metal oxide, a transition metal silicate, a Group 13 metal oxide or a Group 13 metal silicate. (In accordance with the IUPAC Nomenclature of Inorganic Chemistry, Recommendations 1990, Group 13 metals include Al, Ga, In, and Tl and the transition metals occupy Groups 3-12.) Additional examples of the metallic material are nitrogen-containing materials, for example, a nitrogen-containing transition metal oxide, a nitrogen-containing transition metal silicate, a nitrogen-containing Group 13 metal oxide, and a nitrogen-containing Group 13 metal silicate. An example of a nitrogen-containing material is a compound containing Hf, Al, O, and N. The residue can comprise a mixture of two or more of the metallic materials.
Preferably, the metallic material has a high dielectric constant, for example, a constant greater than that of silicon dioxide (that is, greater than about 4.1), more preferably greater than about 5, even more preferably at least about 7. Examples of preferred metallic materials are Al2O3, HfO2, ZrO2, HfSixOy, and ZrSixOy (x is greater than 0 and y is 2x+2), and mixtures thereof.
The residue comprising the metallic material can exist in various forms, for example, as a monolithic coating or in the form of a laminate comprising two or more layers of the metallic material. Exemplary laminates comprise at least two layers of at least one member selected from the group of the following materials: a transition metal oxide, a transition metal silicate, a Group 13 metal oxide, a Group 13 metal silicate, a nitrogen-containing transition metal oxide, a nitrogen-containing transition metal silicate, a nitrogen-containing Group 13 metal oxide, and a nitrogen-containing Group 13 metal silicate. The laminate alternates preferably between at least one of the foregoing materials and, optionally, other materials such as insulating materials. For example, the laminate may be comprised of alternating layers of HfO2 and Al2O3. The laminate may consist also of a certain number of layers of a first material and a certain number of layers of a second material or, alternatively, outer layers of at least one first material and inner layers of at least one second material.
The cleaning agent for use in the practice of the present invention can be any species which is capable of volatilizing the residue which is adhered to the underlying metallic surface, for example, the walls of a reaction chamber. Examples of compounds which are a source of the cleaning agent include boron (B)-containing compounds and compounds containing halogens (F, Cl, Br, and I) such as, for example, HCl, HBr, HI, COCl2, ClF3, and NFzCl3-z, wherein z is an integer from 0 to 3. The preferred source for the cleaning agent is the aforementioned boron-containing compound, for example, BCl3, BBr3, BF3 and a mixture comprising two or more thereof. Among the foregoing, BCl3 is the particularly preferred source of the cleaning agent and BCl is the particularly preferred species to monitor. BCl3 is a liquefied gas at room temperature and can be delivered readily to the site of the cleaning operation, for example, a reaction chamber.
The activated form of the cleaning agent can be formed in any suitable way. For example, it can be formed from only the compound which is the source thereof or from such compound in admixture with one or more inert diluent gases, for example, nitrogen, CO2, helium, neon, argon, krypton, and xenon. An inert diluent gas can be used, for example, to modify the plasma characteristics and cleaning process to better suit a particular application. A gaseous mixture will comprise typically about 1.0 to about 100 vol. % of the “compound” source and about 0 to about 99 vol. % of the inert gas, more typically about 10 to about 50 vol. % of the “compound” source and about 50 to about 90 vol. % of the inert gas.
The cleaning agent may be activated by subjecting one or more compounds comprising the source thereof to one or more energy sources which are effective to activate the compound(s); this can be done in the presence or absence of a diluent gas. Examples of such energy sources include plasma, α-particles, β-particles, γ-rays, x-rays, high energy electron, electron beam sources of energy; ultraviolet (wavelengths ranging from 10 to 400 nm), visible (wavelengths ranging from 400 to 750 nm), infrared (wavelengths ranging from 750 to 105 nm), microwave (frequency >109 Hz), radio-frequency wave (frequency >106 Hz) energy; thermal; RF, DC, arc or corona discharge; sonic, ultrasonic or megasonic energy. A mix of two or more energy sources can be used also.
As may be desired, thermal or plasma activation can be used to improve the efficacy of the cleaning of high dielectric constant residues. For thermal activation, for example, the residue-containing substrate can be heated up to about 600° C., or up to about 400° C., or up to about 300° C. at a pressure, for example, within the range of about 10 m Torr to about 760 Torr or about 1 Torr to about 760 Torr.
The cleaning agent(s) can be formed in situ, that is, at the site containing the residue or at a remote site. By way of example, it is noted that BCl3 plasma can be generated in situ from a mixture of BCl3 and Helium with a 13.56 MHz RF power supply, with RF power density of at least 0.2 W/cm2, or at least 0.5 W/cm2, or at least 1 W/cm2. In situ BCl3 plasma formation can be achieved also at RF frequencies higher and lower than 13.56 MHz to enhance ion assisted cleaning of grounded ALD chamber walls. An exemplary operating pressure is generally in the range of about 2.5 mTorr to about 100 Torr or about 5 mTorr to about 50 Torr, or about 10 mTorr to about 20 Torr. Optionally, one can also combine thermal and plasma enhancement for more effective cleaning of reactor chamber walls.
In alternative embodiments, a remote plasma source can be used in addition to or in place of an in situ plasma to generate the cleaning agent or to generate additional cleaning agent. In these embodiments, the remote plasma source can be generated, for example, by either an RF or a microwave source. In addition, reactions between remote plasma-generated cleaning agents and high dielectric constant materials can be activated/enhanced by heating CVD or ALD chamber components to an elevated temperature, for example, up to about 600° C., or more preferably up to about 400° C., or even more preferably up to about 300° C.
Other means of activation and enhancement to the cleaning process can be employed also. For example, a photon-induced chemical reaction to generate a cleaning agent and enhance the cleaning reaction can be used.
In addition to being thermodynamically favorable, a chemical reaction often requires an external energy source to overcome an activation energy barrier so that the reaction can proceed. The external energy source can be, for example, thermal heating or plasma activation. Higher temperatures can accelerate chemical reactions and make reaction byproducts, for example, a volatilized residue product, more volatile. However, there may be practical limitations on temperature in production deposition chambers. Plasmas can generate more cleaning agents to facilitate reactions. Ions in the plasmas are accelerated by the electric field in the plasma sheath to gain energy. Energetic ions impinging upon surfaces can provide the energy needed to overcome reaction activation energy barrier. Ion bombardment helps to also volatilize and remove reaction byproducts. These are common mechanisms in plasma cleaning and reactive ion etching.
The cleaning agent is preferably maintained in contact with the residue for as long as it takes to volatilize the residue. Such contact time will depend on various factors, for example, the nature of the constituents that comprise the residue, the composition of the plasma cleaning agent, and the thickness and other physical characteristics of the residue. Speaking generally and by way of example, the contact time can fall within the range of about 10 seconds to about 60 minutes depending upon the amount of residue that has coated the chamber walls.
Any suitable means can be used to monitor the amount of each of the volatilized metal and of the plasma cleaning agent in the vapor which contains these materials. Such monitoring can involve either direct or indirect evaluation of the amounts. As explained below in connection with the description of a preferred embodiment of the present invention, the preferred method for the monitoring operation involves the use of optical emission spectroscopy (OES). Other exemplary “monitoring” means are UV-VIS absorption spectroscopy, microwave absorption spectroscopy, near infrared spectroscopy, infrared spectroscopy, and mass spectrometry.
Set forth hereafter is additional information related to a preferred embodiment of the present invention for determining the endpoint of a cleaning process in which the residue is adhered to a surface of metal which includes aluminum, the residue comprises a metallic material of aluminum oxide. BCl is a species created from BCl3 by the plasma, AlCl is a species created by the plasm from AlCl3, which is the volatile form a aluminum chloride from the substrate surface.
In forming in situ a plasma from BCl3, the predominant reactive species in the bulk of the plasma is BCl (boron monochloride). Without being bound by a particular theory, it is believed that BCl can be produced via electron-molecule collisions in the plasma pursuant to the following reaction
BCl3+e−→BCl+Cl2+e− (1)
or by dissociative ionization according to the following reactions
BCl3+e−→BCl+Cl2++2e− and (2)
BCl3+e−→BCl++Cl2+2e−. (3)
BCl+ ions can recombine with electrons to form excited BCl according to the reaction
BCl++e−→BCl*. (4)
In addition, ground state BCl can be excited directly by collision with electrons pursuant to the reaction
BCl+e−→BCl*+e−. (5)
The excited BCl* can give off by radiation its energy and return to ground state via the reaction
BCl*→BCl+hv. (6)
The optical emission process shown in equation (6) gives the characteristic emission of BCl A 1Π−×1Σ+ emission at 272 nm.
Under higher resolution, this optical emission spectrum can be resolved into a triplet structure with three peaks at 272.00, 272.17, and 272.22 nm respectively. These are emissions from different ro-vibrational bands. For the purpose of this invention, the unresolved peak intensity at lower spectral resolution can be used. Alternatively, one of the resolved triplet peak intensity at higher spectral resolution can be used or the average intensity of the three resolved peaks can be used.
Among the dissociative fragments of BCl3 in a plasma, BCl appears to be the most effective agent to react with high dielectric constant materials, for example, Al2O3, HfO2, and ZrO2. Without being bound by a particular theory, it is believed that BCl is particularly effective because there is a synergistic reaction between two chemical processes, namely the removal of oxygen that assists the breaking of metal oxygen bonds and the formation of volatile metal chlorides, for example,
Al2O3+BCl→AlCl3+B2O3, (7)
HfO2+BCl→HfCl4+B2O3, and (8)
ZrO2+BCl→ZrCl4+B2O3. (9)
Without being bound the a particular theory when the metallic material residue is removed from the metal surface to which it is adhered (for example, the walls of a reactor chamber), the metal surfaces of the chamber are exposed to the plasma. Aluminum alloy, particularly aluminum (6061), is one of the most common materials of construction for deposition chambers. When aluminum is exposed to a plasma formed from BCl3, etching reactions occur, for example,
Al+BCl→AlCl3+B. (10)
Similar to BCl, AlCl radicals can be excited also to higher energy states, for example, by the reaction
AlCl3+e−→AlCl*+Cl2+e−. (11)
The excited AlCl* then undergoes spontaneous radiative decay A 1Π−×1Σ+ at 261.44 nm.
The reaction between aluminum alloy and BCl, for example, equation (10) above, proceeds at a higher rate than the reactions between the metallic material residue and BCl, for example, equations (7) through (9) above. Although the production of BCl is relatively constant via reactions (1) through (3) above, the higher etch rate of the aluminum alloy in reaction (10) results in a lower BCl density in the plasma. In addition, the higher rate of reaction in the etching of aluminum results also in an increase in the density of AlCl in the plasma. The increase in the AlCl density in the plasma is not particularly dramatic because of the initial presence in the plasma of AlCl as a result of the volatilization of the Al-containing residue. However, in a process in which the residue does not contain aluminum, for example, hafnium oxide, the sudden surge of AlCl in the plasma is indeed dramatic as the underlying aluminum metal is etched and volatilized by the plasma upon the removal of the residue and contact therewith.
The changes in the chemical compositions of the plasma as the cleaning process approaches its endpoint induce changes in the optical emission spectra of the components comprising the plasma. Before reaching the endpoint, the relatively high BCl density leads to stronger optical emission intensity at its characteristic 272 nm. As the cleaning process approaches its endpoint, lower BCl density leads to weaker BCl optical emission at 272 nm. At the same time, the emergence or the increase of AlCl density in the plasma leads to appearance or increase of AlCl optical emission at 261 nm. Therefore, changes in the intensities of the characteristic optical emission features of BCl at 272 nm and AlCl at 261 nm can be used as an indicator of the endpoint of the cleaning process.
Thus, a preferred embodiment of the present invention includes monitoring the amount of each of the volatilized metal and the plasma cleaning agent by the use of optical emission spectroscopy (OES). The plasma constituents (including BCl, volatilized residue products, and volatilized metal chlorides) are excited continuously by electrons and collisions in the plasma and give off emissions ranging from ultraviolet to infrared radiation as they relax to a lower energy state. An optical emission spectrometer is used to diffract the emissions into component wavelengths, for example, BCl emission at 272 nm and volatilized metal chlorides, for example, AlCl emission at 261 nm. The optical emission spectrometer is used also to determine the intensity of the AlCl emission and the intensity of the BCl emission, each of which is proportional to the concentration of each species. A ratio of the intensity of the AlCl emission to the intensity of the BCl emission is monitored. The endpoint is reached when the ratio increases from a lower to a higher value.
In general, determining the ratio based on the intensity of a constituent whose emission intensity is increasing (for example, AlCl) and the intensity of a constituent whose emission intensity decreasing (for example, BCl) towards the endpoint of the cleaning process enhances greatly the detection sensitivity. This is an improvement relative to the use only of the intensity of one of the OES peaks or the ratio between an OES peak and the OES background, or the ratio between one OES peak from a reactive species (for example, F) and an inert background gas (for example, argon actinometer).
Emission intensity can be detected in OES by utilizing a multi-channel detector, for example, a charge-coupled device (“CCD”) or photodiode array (“PDA”), which has the advantage of simultaneous detection of all the spectral features. A scanning type spectrometer to record OES can be used also. Alternatively, a combination of narrow band filters and photo detector to selectively detect I(AlCl) and I(BCl) intensities can be used. For example, in certain embodiments, a narrow band filter centered at 261 nm with a full width at half maximum (FWHM) of 3 nm may be used to select an AlCl emission and a narrow band filter centered at 272 nm with a FWHM of 3 nm to select BCl emission. The selected optical emission can be detected by a photon sensor, for example, a photodiode or a photomultiplier tube. The use of narrow band filter/photodiode combination may offer one or more of the following advantages: low cost, field robust, rapid response, and/or ease of integration into the process reactor for automatic endpoint detection and process control. The center wavelength and the bandwidth of the spectral filters should be selected to capture the maximum intensity of the desired peak without interference from nearby unwanted peaks.
In certain embodiments, data manipulation methods, for example, using the derivative of the intensity ratio I(AlCl)/I(BCl) can also be applied to locate the cleaning endpoint. Furthermore, alternative detection methods such as mass spectrometry are useful for detecting AlCl and BCl when alternative methods for activating the cleaning process such as remote plasma and thermal or UV activation are used.
Additional objects, advantages, and novel features of this invention will become apparent to those skilled in the art upon examination of the following examples thereof, which are not intended to be limiting.
The following examples are designed to represent conditions that are present during the cleaning of a reaction chamber with a BCl plasma cleaning agent. Test coupons made of aluminum metal simulate the metal surface of the reaction chamber. Test coupons that are covered with an Al2O3 film simulate a metallic material residue on the surface of a reaction chamber. The test coupons are placed on an RF-powered lower electrode in a modified parallel plate Gaseous Electronics Conference (GEC) reactor and exposed to a BCl plasma cleaning agent which is formed from BCL3. The plasma cleaning agent reacts with the Al2O3 film and the aluminum metal of the test coupons to produce respectively a volatilized residue product (AlCl) and a volatilized metal (AlCl).
Optical emission spectroscopy is used to monitor the presence of AlCl and BCl in the GEC reactor. Optical emission spectra (OES) are recorded in a capacitatively coupled BCl plasma. The plasma conditions are: 10 sccm BCl3 flow, 500 mTorr chamber pressure, and 100 W RF power at 13.56 MHz. OES are recorded by an optical fiber coupled charge-coupled device (CCD) array spectrometer (Ocean Optics S2000). The OES peak which is associated with the presence of A1C1 appears at the wavelength of 261 nm. The OES peak which is associated with the presence of BCl appears at the wavelength of 272 nm. The OES intensity which is representative of the amounts of AlCl and BCl present is given in arbitrary units.
The OES of each of a test coupon coated with an Al2O3 film and of an uncoated test coupon is compared. In the first run, a test coupon coated with an Al2O3 film by atomic layer deposition (ALD) is loaded into the GEC reactor. OES are recorded as the Al2O3 film is removed from the surface of the coupon by the cleaning agent. In the second run, an uncoated aluminum metal sample is loaded into the reactor. OES are recorded as the aluminum metal is etched by the BCl cleaning agent. In each of the runs, OES are recorded also for BCl.
As shown in
Table 1 lists the OES intensities and the intensity ratio between the AlCl emission at 261 nm and the BCl emission at 272 nm for these two runs.
As shown in Table 1, the I(AlCl)/I(BCl) ratio is 0.67 when etching the Al2O3 film and 1.26 when etching the aluminum metal sample. Some of the reactor internal components (e.g. showerhead) are made of aluminum alloy. This contributes to the baseline AlCl peak level in the data. For a reaction chamber having internal surfaces completely coated with an Al2O3 residue, the contrast in OES intensity ratio of I(AlCl)/I(BCl) will be more significant as the Al2O3 residue is removed and the more reactive aluminum alloy is exposed to BCl plasma.
The time evolution of I(AlCl) and I(BCl) and their ratio are measured to monitor the progression of a cleaning process. A test coupon coated with an Al2O3 film is loaded into the reactor as described above. The plasma recipe is the same as for Example No. 1. OES is continuously recorded when the BCl plasma is turned on.
The foregoing examples and description of the preferred embodiments should be taken as illustrating, rather than as limiting the present invention as defined by the claims. As will be readily appreciated, numerous variations and combinations of the features set forth above can be utilized without departing from the present invention as set forth in the claims. Such variations are not regarded as a departure from the spirit and scope of the invention, and all such variations are intended to be included within the scope of the following claims.
