On-line cleaning method for CVD vaporizers

A direct liquid injection system and process which has on-line cleaning of the vaporizers without the need for shutting down the CVD process and thus eliminating down time is provided. The cleaning process includes the steps of providing at least one metalorganic precursor to a first vaporizer to produce a vapor containing the at least one precursor; transporting the vapor to a deposition chamber; periodically interrupting the supply of the at least one metalorganic precursor to the first vaporizer; providing the at least one metalorganic precursor to a second vaporizer to produce a vapor containing the at least one precursor; transporting the vapor to the deposition chamber; and during at least a portion of the time when the supply of the metalorganic precursor is interrupted to the first vaporizer, providing a cleaning fluid (either liquid solvent or cleaning gas plasma) to the first vaporizer, which fluid is effective to at least partially remove deposits of the metalorganic precursor. The process may be either carried out as a batch process, or more preferably, as a continuous process to avoid the need to shut down the system.

BACKGROUND OF THE INVENTION
 This invention relates to a direct liquid injection system which provides
 for on-line cleaning, and more particularly to a system and process for
 the controlled deposition of metal oxide layers using chemical vapor
 deposition techniques.
 Semiconductor devices such as dynamic random access memories (DRAMs) have
 undergone substantial decreases in size and increases in charge storage
 density over the past several years, and it is expected that these trends
 will continue into the future. In order to increase capacity while
 decreasing size, DRAM designs have become increasingly complex. One
 problem has been the design of capacitors in the DRAM which will hold the
 necessary electrical charge representing stored data.
 Oxides of silicon have conventionally been used as the dielectric materials
 in such DRAM capacitors. Silicon oxides, however, have relatively low
 dielectric constants and limited charge storage densities. Accordingly,
 there has been an effort in the art to identify materials having higher
 dielectric constants which are suitable for use in DRAM designs. Interest
 in ferroelectric materials such as barium strontium titanates (Ba.sub.1-x
 Sr.sub.x TiO.sub.3, known as BSTs) and lead zirconate titanates
 (Pb(Zr.sub.x Ti.sub.1-x)O.sub.3, known as PZTs) has grown because such
 materials have relatively higher dielectric constants than silicon oxides,
 are structurally stable, and can be prepared using known techniques.
 Of the many chemical and physical deposition techniques used in the art to
 form thin film layers of such ferroelectric materials, metal organic
 chemical vapor deposition (MOCVD) using direct liquid injection appears to
 hold the most promise. For example, Desu et al, U.S. Pat. Nos. 5,431,958
 and 5,527,567 teach MOCVD techniques, including direct liquid injection
 ('567 patent), to provided layered ferroelectric films for the manufacture
 of capacitors. Si et al, U.S. Pat. No. 5,629,229, also teach the
 manufacture of DRAMs using MOCVD techniques.
 In MOCVD, the metalorganic precursors which are used are dissolved in
 liquid solvents which are then pumped in precise proportions to a
 vaporizer. The vaporized precursors are then sent to a CVD chamber where
 they are deposited on a substrate. The composition and properties of the
 deposited films of the ferroelectric materials are highly dependent on the
 ability of the direct liquid injection system to supply the correct
 proportions of precursors to the vaporizer and thence to the CVD chamber.
 Other variables in the system will also affect the composition and
 properties of the deposited films and include the condition of the
 vaporizer, the temperature of the vaporizing surfaces in the vaporizer,
 the concentrations of the metalorganic precursors in the solvent, and
 local temperature variations on the substrate surface in the reactor. The
 local temperature variations as well as accumulation of precursor
 compounds on the surfaces of the vaporizer both affect vaporization
 efficiency and can cause fluctuations in the composition and properties of
 the films which are deposited.
 Such accumulations of precursor compounds and oxidative reaction products
 of the precursor compounds have the tendency to build-up over time and
 clog both the outlet to the vaporizer as well as the direct liquid
 injection mechanism. This leads not only to undesirable variations in the
 ratios of the precursor compounds which are deposited, but also to
 possible clogging and shutdown of the deposition process. To address these
 problems, Gardiner et al, U.S. Pat. No. 5,362,328, teach the use of a
 cleaning subsystem in a chemical vapor deposition process in which a
 solvent is supplied to the vaporizer to solubilize any deposited compounds
 and flush them away. However, the Gardiner et al cleaning subsystem
 requires that the CVD process be periodically shut down during the
 cleaning cycle. Accordingly, the need still exists in this art for a
 direct liquid injection system which provides for on-line cleaning without
 the need for shutting down the CVD process.
 SUMMARY OF THE INVENTION
 The present invention meets that need by providing a direct liquid
 injection system which has on-line cleaning of the vaporizers without the
 need for shutting down the CVD process, and thus eliminating down time.
 The present invention also provides alternative sources of cleaning fluid
 which may be selected to remove metalorganic precursor and oxidation
 product residues which are deposited in the vaporizers.
 In accordance with one aspect of the present invention, a process for the
 on-line cleaning of a direct liquid injection system is provided and
 includes the steps of providing at least one metalorganic precursor to a
 first vaporizer to produce a vapor containing the at least one precursor;
 transporting the vapor to a deposition chamber; periodically interrupting
 the supply of the at least one metalorganic precursor to the first
 vaporizer; providing the at least one metalorganic precursor to a second
 vaporizer to produce a vapor containing the at least one precursor;
 transporting the vapor to the deposition chamber; and during at least a
 portion of the time when the supply of the metalorganic precursor is
 interrupted to the first vaporizer, providing a cleaning fluid to the
 first vaporizer, which fluid is effective to at least partially remove
 deposits of the metalorganic precursor and oxidation products. Preferably,
 the cleaning fluid is effective to remove substantially all of the
 deposits and residue of the at least one metalorganic precursor and any
 oxidation products which may have formed.
 The process may be either carried out as a batch process, or more
 preferably, as a continuous process. Thus, when the supply of metalorganic
 precursor is interrupted to the first vaporizer for cleaning, the supply
 to the second vaporizer is initiated so that there is a continuous flow of
 vaporized precursor being supplied to the deposition chamber. That supply
 of metalorganic precursor is maintained to the second vaporizer until a
 buildup of deposits or residue is detected. Then the procedure is reversed
 by resuming the supply of the at least one metalorganic precursor to the
 first vaporizer and interrupting the supply of the at least one
 metalorganic precursor to the second vaporizer. During at least a portion
 of the time when the supply of the metalorganic precursor is interrupted
 to the second vaporizer, a cleaning fluid is provided to the second
 vaporizer, which fluid is effective to at least partially remove deposits
 of the metalorganic precursor and oxidation products, and preferably, is
 effective to substantially completely remove deposits and residues of the
 metalorganic precursor and any oxidation products which may have formed.
 In a preferred form, the at least one metalorganic precursor is dissolved
 in a liquid solvent carrier which is supplied to the vaporizers. The
 cleaning fluid may comprise a liquid solvent for the at least one
 metalorganic precursor. The cleaning fluid may be recovered and recycled
 after it has been passed through the vaporizer. Alternatively, the
 cleaning fluid may comprise a gas plasma which is an etchant for the
 deposits of metalorganic precursor and oxidation products. The gas plasma
 may be formed in a conventional manner, such as, for example, using
 microwave energy to form the plasma. The gas plasma is formed from an
 etchant gas which is preferably selected from the group consisting of
 NF.sub.3, CIF.sub.3, and HF. The plasma is formed at a temperature and at
 a pressure which permits it to be sufficiently long-lived to effect its
 cleaning function in the vaporizers.
 Additionally, the vaporizer may be cleaned by sequentially supplying
 different cleaning fluids to it. Thus, the vaporizer may be cleaned by
 first using a gas plasma which is then followed by a solvent-containing
 fluid. Alternatively, the vaporizer may be cleaned by first using a
 solvent-containing cleaning fluid which is followed by a gas plasma
 treatment.
 The step of monitoring the build up of deposits in the first vaporizer and
 interrupting the supply of the metalorganic precursor is preferably
 designed to operate when such buildup reaches a predetermined level. The
 degree of buildup of metalorganic precursor deposits may be measured by
 monitoring the flow rate of the vapor from the first vaporizer. When such
 flow rate drops below a predetermined level, the supply of metalorganic
 precursor is interrupted, and cleaning fluid is sent to the vaporizer.
 The present invention also provides an apparatus for a direct liquid
 injection system with on-line cleaning which includes a source of at least
 one metalorganic precursor; first and second vaporizers for the at least
 one metalorganic precursor; a chemical vapor deposition chamber for
 receiving vaporized metalorganic precursor; a source of cleaning fluid for
 removing deposits of the at least one metalorganic precursor from the
 first and second vaporizers; and a controller for directing the at least
 one metalorganic precursor to either the first or second vaporizer, for
 periodically interrupting the flow of the at least one metalorganic
 precursor to the first or second vaporizer, and for initiating a flow of
 the cleaning fluid to the vaporizer which has had its supply of
 metalorganic precursor interrupted. The system also preferably includes
 monitors in or adjacent to the first and second vaporizers for monitoring
 the build up of deposits of the at least one metalorganic precursor. The
 monitors may comprise flow rate measurement devices.
 In a preferred form, a valve is positioned between the source of the at
 least one metalorganic precursor and the first vaporizer, and another
 valve is positioned between the source of the at least one metalorganic
 precursor and the second vaporizer. The controller regulates the operation
 of the valves by feedback control from the monitors.
 The present invention has the advantage of being able to operate
 continuously while providing cleaning fluids which are suitable and
 effective to remove even the most difficult to remove deposits and
 residues of the metalorganic precursors and any oxidation products which
 may have formed. These and other features and advantages of the invention
 will become apparent from the following detailed description, the
 accompanying drawings, and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention provides a system and process for using direct liquid
 injection to deposit single or multicomponent oxide films on semiconductor
 devices. Examples of multicomponent oxides include, but are not limited
 to, barium strontium titanate (BST), bismuth strontium tantalum oxide
 (SBT), lead zirconate titanate (PZT), and lanthanum lead zirconate
 titanate (PLZT). These ferroelectric materials exhibit high dielectric
 constants and low current leakage, are chemically and physically stable,
 and can be deposited in a dense film at high deposition rates using
 conventional techniques to provide good step coverage of substrates. As
 such, these materials find use in capacitor structures in semiconductor
 devices such as random access memories as well as in other devices using
 ferroelectric materials including pyroelectric detectors, ultrasonic
 sensors, and electro-optic devices including optical switches and optical
 displays. As used herein, the term "substrate" means any material with
 sufficient load-bearing capability and internal strength to withstand the
 application of a layer or layers of additional materials during a
 fabrication process. As used herein, the term encompasses silicon
 structures such as silicon wafers and semiconductor devices, including
 semiconductor devices in the process of fabrication. The term "silicon
 wafer" means either the lowest layer of semiconductor material in a wafer
 or a wafer having additional layers or structures formed thereon.
 The metalorganic precursors used in the practice of the present invention
 are preferably alkyls, alkoxides, .beta.-diketonates, or metallocenes of
 the corresponding metal elements which make up the multicomponent metal
 oxide films. Suitable metalorganic precursors for use in a MOCVD process
 include those compounds which exhibit high vapor pressure at low
 vaporization temperatures, low decomposition temperatures, a large
 "window" between vaporization and decomposition temperatures, no
 contamination from organic constituents of the precursors, stability under
 ambient conditions, and nontoxicity. These properties of the precursors
 may be modified and adjusted to some extent by the choice of the
 particular organic substituents. For example, volatility of a metal
 .beta.-diketonate can be varied by varying the alkyl group to which it is
 attached.
 As shown in the drawing FIGURE, the system 10 includes a source 12 of at
 least one metalorganic precursor. While a single source is shown for
 purposes of illustration, it will be appreciated that the number of
 precursors may vary depending upon the particular metal oxide film which
 is to be formed. For example, there may be three or more separate sources
 of different metalorganic precursors. In such a case, as described in
 commonly-assigned copending U.S. patent application Ser. No. 09/037,235,
 filed Mar. 10, 1998, the disclosure of which is hereby incorporated by
 reference, the individual metalorganic precursors are preferably supplied
 in liquid form by dissolving them in suitable liquid carriers. The
 separate streams of liquid precursors are then mixed prior to being
 supplied to the vaporizer. Suitable solvent carriers for metalorganic
 compounds include tetrahydrofuran (C.sub.4 H.sub.8 O), isopropanol
 (C.sub.3 H.sub.7 OH), tetraglyme (C.sub.10 H.sub.22 O.sub.5), and mixtures
 thereof. Typically, the concentration of metalorganic precursors in the
 solvent carrier will be between about 0.01 to about 1.0 mole per liter of
 solvent.
 The metalorganic precursor, or blend of metalorganic precursors, is sent to
 first vaporizer 14. Vaporizer 14 is preferably one which can quickly heat
 the precursor (preferably to at least 250.degree. C.) and efficiently
 cause the precursor and liquid carrier to flash vaporize. One suitable
 vaporizer is commercially available from MKS Instruments of Andover, Mass.
 and utilizes heated metal disks. Vaporizer 14 may also include a port (not
 shown) for mixing an inert gas such as nitrogen with the vaporized
 precursor to carry it into the chemical vapor deposition chamber. The
 inert gas which is supplied to vaporizer 14 may also be used to vary the
 composition, and thus the vaporizing characteristics, of the precursor.
 After vaporization, the metalorganic precursor is sent to chemical vapor
 deposition (CVD) chamber 16. Chamber 16 may be either a hot wall or cold
 wall MOCVD reactor of the type conventionally used in this art. A
 substrate (not shown) is placed into chamber 16 which is heated to a
 temperature of between about 300.degree. to 800.degree. C. and maintained
 at a pressure of between about 1.0.times.10.sup.-4 to 100 torr. In chamber
 16, the vaporized metalorganic precursor(s) decompose and are deposited as
 single or multi-component metal oxides on the surface of the substrate.
 Typically, multi-component oxide films are deposited to a thickness of
 from between about 10 to about 1000 .ANG., and more preferably from about
 50 to about 300 .ANG.. Such deposition takes from about 30 seconds to
 about 20 minutes, and more typically from about 1 to 2 minutes.
 While the system is designed to supply precise quantities of metalorganic
 precursors to vaporizer 14, and thence to chamber 16, because of the
 conditions in vaporizer 14, there is a tendency for the metalorganic
 precursor to deposit and solidify on the surfaces of the vaporizer.
 Further, some oxidation products of the precursor may have formed as well.
 Accumulation of precursor and oxidation compounds on the surfaces of the
 vaporizer, including the injector and vapor outlet, both affect
 vaporization efficiency and can cause fluctuations in the composition and
 properties of the films which are deposited.
 To insure that the vaporized precursor stream continuously supplies the
 correct amount of vaporized precursor to chamber 16 for deposition and,
 for multicomponent embodiments, maintains its desired stoichiometric ratio
 of components, the vaporized stream is continuously (or intermittently)
 monitored by monitoring device 18 which, in the illustrated embodiment, is
 located downstream from first vaporizer 14. It will be appreciated,
 however, that depending on the particular type of monitoring device, the
 device may be located in or adjacent to the vaporizer. Monitoring device
 18 may monitor any of a number of physical or process parameters of the
 vaporized precursor stream. A suitable monitoring device is a flow rate
 monitor for measuring the volumetric flow rate of the precursor. Monitor
 18 supplies information to a controller 22. Controller 22 may be either a
 programmable logic controller or a programmable general purpose computer.
 When that flow rate is measured to have dropped below a predetermined
 value, the drop in flow indicates a buildup of deposits and/or residues on
 the surfaces of the vaporizer. In the prior art, such a buildup
 necessitated the shutdown of the system for periodic cleaning of the
 vaporizer surfaces. However, the system of the present invention is
 preferably designed to operate continuously. For that purpose, a second
 vaporizer 20 is provided. When monitor 18 supplies information to
 controller 22 which indicates such a buildup, controller 22 acts to
 interrupt the flow of metalorganic precursor to first vaporizer 14 by
 closing valve 24. Simultaneously, controller 22 opens valve 26 to redirect
 the supply of metalorganic precursor(s) to second vaporizer 20.
 Controller 22 may also at this time, or a later point in time, begin the
 cleaning cycle for first vaporizer 14 to remove the buildup of deposits
 and/or residues. A cleaning fluid in the form of either a gas or a liquid
 may be utilized. For example, a liquid solvent for the metalorganic
 precursor(s) and oxidation products may be stored in reservoir 28 and its
 flow to first vaporizer 14 may be initiated by opening valves 30 and 32,
 respectively. A series of pumps (not shown) insures the proper flow of all
 components through the system. The solvents may be the same solvents
 described above as carriers for the metalorganic precursor(s). Spent
 solvent, along with the metalorganic precursor deposits and residues which
 have been removed from vaporizer 14, is taken from first vaporizer 14
 through line 34 and condenser 36 to spent solvent reservoir 38. The spent
 solvent may either be disposed of or sent to a recovery station 40 where
 impurities and contaminants are removed from the solvent, and the solvent
 is recycled to solvent reservoir 28 for reuse.
 Alternatively (or in addition to the use of liquid solvent), the cleaning
 fluid may be a gas plasma using a cleaning gas which is an etchant for the
 metalorganic precursor deposits and residues in the vaporizer. It has been
 found that a cleaning gas plasma is effective to react with and remove
 deposits which are difficult to remove completely using liquid solvents.
 The gas plasma is generated in generator 42 using a cleaning gas from
 source 44. Controller 22 opens valve 46 to initiate the flow of plasma to
 vaporizer 14. Thus, the present invention contemplates that the vaporizer
 may be cleaned using a liquid solvent, a gas plasma, or a sequence in
 which both cleaning fluid are utilized in series.
 The gas plasma may be formed in a conventional manner, such as, for
 example, using microwave energy to form the plasma. The gas plasma is
 formed from an etchant gas which is preferably selected from the group
 consisting of NF.sub.3, CIF.sub.3, and HF. The plasma is formed at a
 temperature and at a pressure which permits it to be sufficiently
 long-lived to effect its cleaning function in the vaporizers. The spent
 gas plasma and removed deposits of metalorganic precursor(s) are sent
 through line 34 for disposal or recovery.
 Once the deposits and residues are cleaned from vaporizer 14, controller 22
 shuts off the flow of cleaning fluid. The flow of vaporized metalorganic
 precursor from second vaporizer 20 is monitored by monitoring device 48,
 located downstream from second vaporizer 20, which reports information to
 controller 22. Once a buildup of deposits in second vaporizer 20 is
 detected, controller 22 resumes the supply of metalorganic precursor to
 first vaporizer 14, and interrupts the flow of metalorganic precursor to
 second vaporizer 20. Then, the cleaning cycle for second vaporizer 20 is
 initiated by beginning the flow of either liquid solvent from reservoir 28
 by opening valve 50, or, alternatively, gas plasma is introduced into
 second vaporizer 20 by opening valve 52. The cleaning cycle for second
 vaporizer is as was previously described with respect to first vaporizer
 14.
 Thus, the system and process of the present invention may be operated in a
 batch process. However, most preferably, the process and system are
 operated to provide a continuous supply of vaporized metalorganic
 precursor species to CVD chamber 16. This is accomplished by providing
 on-line cleaning of vaporizer chambers while maintaining the flow of
 metalorganic precursor to the CVD chamber.
 While certain representative embodiments and details have been shown for
 purposes of illustrating the invention, it will be apparent to those
 skilled in the art that various changes in the methods and apparatus
 disclosed herein may be made without departing from the scope of the
 invention, which is defined in the appended claims.