Patent Publication Number: US-2010129548-A1

Title: Ald apparatus and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/561,758 filed on Mar. 8, 2007, which is the US national stage filing of PCT Application No. PCT/US04/020630 filed Jun. 28, 2004, which claims the benefit of U.S. Provisional Application No. 60/483,152 filed on Jun. 27, 2003. The foregoing applications are hereby incorporated by reference to the same extent as though fully disclosed herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of atomic layer deposition (“ALD”), and more particularly to apparatus and methods for performing ALD with high throughput and low cost. 
     BACKGROUND OF THE INVENTION 
     Thin film deposition is commonly practiced in the fabrication of semiconductor devices and many other useful devices. An emerging deposition technique, atomic layer deposition (ALD), offers superior thickness control and conformality for advanced thin film deposition. ALD is practiced by dividing conventional thin-film deposition processes into single atomic-layer deposition steps, named cycles, which are self-terminating and deposit precisely one atomic layer when conducted up to or beyond self-termination exposure times. The deposition per cycle during an ALD process, the atomic layer, typically equals about 0.1 molecular monolayer to 0.5 molecular monolayer. The deposition of atomic layer is the outcome of a chemical reaction between a reactive molecular precursor and the substrate. In each separate ALD reaction-deposition step, the net reaction deposits the desired atomic layer and eliminates the “extra” atoms originally included in the molecular precursor. 
     In ALD applications, typically two molecular precursors are introduced into the ALD reactor in separate stages. Adequate ALD performance requires that different molecular precursors are not allowed to intermix within the deposition chamber, at the same time. Accordingly, the reaction stages are typically followed by inert-gas purge stages that eliminate the molecular precursors from the chamber prior to the separate introduction of the other precursor. 
     During the ALD process, films can be layered down in equal metered sequences that are all identical in chemical kinetics, deposition per cycle, composition, and thickness. This mechanism makes ALD insensitive to transport nonuniformity resulting in exceptional thickness control, uniformity and conformality. 
     If ALD is to become commercially practical an apparatus capable of changing the flux of molecular precursors from one to the other abruptly and fast needs to be available. Furthermore, the apparatus must be able to carry this sequencing efficiently and reliably for many cycles to facilitate cost-effective coating of many substrates. A useful and economically feasible cycle time must accommodate a thickness in a range of about from 3 nm to 30 nm for most semiconductor applications, and even thicker films for other applications. Cost effectiveness dictates that substrates be processed within 2 minutes to 3 minutes, which means that ALD cycle times must be in a range of about from 5 seconds to 0.5 seconds and even less. Multiple technical challenges have so far prevented cost-effective implementation of ALD systems and methods for manufacturing of semiconductor devices and other devices. 
     Given the need for short cycle times, chemical delivery systems suitable for use in ALD must be able to alternate incoming molecular precursor flows and purges with sub-second response times. The need to achieve short cycle times requires the rapid removal of these molecular precursors from the ALD reactor. Rapid removal in turn dictates that gas residence time in the ALD reactor be minimized. Gas residence times, τ, are proportional to the volume of the reactor, V, the pressure, P, in the ALD reactor, and the inverse of the flow, Q, τ=VP/Q. Accordingly, lowering pressure (P) in the ALD reactor facilitates low gas residence times and increases the speed of removal (purge) of chemical precursor from the ALD reactor. In contrast, minimizing the ALD reaction time requires maximizing the flux of chemical precursors onto the substrate through the use of a high pressure within the ALD reactor. In addition, both gas residence time and chemical usage efficiency are inversely proportional to the flow. Thus, while lowering flow will increase efficiency, it will also increase gas residence time. 
     Existing ALD apparatuses have struggled with the trade-off between the need to shorten reaction times and improve chemical utilization efficiency, and on the other hand, the need to minimize purge-gas residence and chemical removal times. Thus, a need exists for an ALD apparatus that can achieve short reaction times and good chemical utilization efficiency, and that can minimize purge-gas residence and chemical removal times. 
     Existing ALD apparatuses have also struggled with performance deterioration caused by extensive growth of inferior films on the walls of the ALD chambers. This performance deterioration facilitated short equipment uptime and high cost of maintenance. Thus, a need exists for an ALD apparatus that can minimize the growth of deposits and minimize their impact on performance therefore facilitating substantially longer uptime and reduce the cost of maintenance. 
     Existing ALD apparatuses have struggled with performance deterioration related to slit-valve induced asymmetry with its unavoidable dead-leg cavity. The art of single wafer deposition presents a variety of effective remedies for this problem. For example, U.S. Pat. No. 5,558,717 teaches the advantageous implementation of an annular flow orifice and an annular pumping channel. This annular design requires a relatively wide process-chamber design. In another example, U.S. Pat. No. 6,174,377 describes an ALD chamber designed for wafer loading at a low chuck position, while wafer processing is carried out at a high chuck position, leaving the wafer transport channel, and the flow disturbances associated with it, substantially below the wafer level. Both of these prior art solutions and other prior art solutions are not ideally suited to resolve the slot valve cavity problem in ALD systems. 
     A better solution implements a ring-shaped slit-valve that creates a substantially symmetric chamber environment. Such embodiment is described in U.S. Pat. No. 6,347,919. However, the ring slit-valve described in U.S. Pat. No. 6,347,919 presents significant performance deterioration that is associated with the presence of unprotected elastomeric seals and the respective crevices between the slide of the ring slit-valve and the chamber wall that is notorious for entrapment of chemicals and the growth of deposits and particulates on the seal and within the crevices. While deterioration of chamber performance related to growth of deposits on slit-valve seals is a universal problem with all existing designs of slit valves, ring-shaped slit-valves as taught in U.S. Pat. No. 6,347,919 substantially aggravate that problem due to substantially longer seals and crevices. Unfortunately, this performance limitation makes the ring-shaped slit-valve that was taught in U.S. Pat. No. 6,347,919 practically unusable for ALD applications. 
     A substantial improvement that makes ring-shaped and other perimeter slit-valves suitable and advantageous for ALD applications is described in U.S. Pat. No. 6,911,092 by the inventor of this invention that provides seal and crevice protection during the ALD chemical dose steps therefore making perimeter slit-valves suitable for Synchronously Modulated Flow-Draw ALD apparatus and method. 
     Chemical delivery into ALD chambers has been generally been limited to chemicals with substantial vapor pressure. However, many advantageous ALD films rely on molecular precursors that are substantially non-volatile. Accordingly existing ALD systems have struggled with the challenge of consistent chemical delivery of low-volatility molecular precursors as abruptly shaped doses for promoting high productivity ALD processes. 
     In previous patents and patent application publications by the inventor of this invention, namely, U.S. Pat. No. 6,911,092 and PCT Application Publication No. WO03/062490, embodiments that helped solve some of the problems described above were disclosed. Systems, apparatuses, and methods in accordance with that invention provide Synchronous Modulation of Flow and Draw (“SMFD”) in chemical processes, and in particular, in atomic layer deposition processes and systems. These patents and patent application publications are included here as references. 
     Atomic layer deposition (“ALD”) is preferably practiced with the highest possible flow rate through the deposition chamber during purge, and with the lowest possible flow rate during dosage of chemicals. Accordingly, an efficient ALD system in accordance with U.S. Pat. No. 6,911,092 and PCT Application Publication No. WO03/062490 is able to generate and accommodate significant modulation of flow rates. Under steady-state conditions, the flow of process gas (either inert purge gas or chemical reactant gas) into a chamber, referred to herein as “flow”, substantially matches the flow of gas out of a chamber, referred to herein as “draw”. 
     An important aspect of an embodiment in accordance with the invention described in U.S. Pat. No. 6,911,092 and PCT Application Publication No. WO03/062490 is that it resolves the trade-off in conventional ALD systems between the contradictory requirements of a high flow rate during a purge of the deposition chamber and of a low flow rate during chemical dosage. SMFD in accordance with that invention provides the ability to purge a process chamber at a low-pressure and a high purge-gas flow rate, and sequentially to conduct chemical dosage in the process chamber at a high-pressure and a low flow rate of chemical reactant gas, and to modulate pressures and gas flow rates with fast response times. 
     While the SMFD-ALD device disclosed in the prior applications of this inventor is a significant improvement in the deposition art, in some respects the design still reflects the technology available in conventional deposition processes. It would be highly useful to have an improved process chamber that takes better advantage of the SMFD process, and more fully develops chemical utilization efficiencies. It also would be useful to provide SMFD-ALD chemical source designs better adapted to the SMFD process, especially for the efficient and consistent delivery of vapor from low-volatility liquid and solid chemicals. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments in accordance with the invention provide improved apparatus and method to further enhance the advantages of SMFD-ALD apparatus and method. In one embodiment, the invention provides a method of conducting atomic layer deposition with enhanced material utilization efficiency. In that method, chemical dosage is conducted in a dose and hold mode. During the hold mode, the flow of chemical into the process chamber is terminated while the flux is effectively maintained. This mode is beneficial for the final stages of chemical dose where chemical depletion is minimal, while maintaining chemical flux can further promote the reaction far into saturation to much improve film quality. 
     In one aspect a method in accordance with the invention comprises conducting a chemical dose stage. The chemical dose stage includes firstly flowing a chemical reactant gas to substantially fill up a deposition chamber, secondly the stage includes substantially reducing the flow of the chemical reactant gas while concurrently reducing the flow out of the deposition chamber by increasing the pressure downstream to the deposition chamber to substantially match the flow out of the deposition chamber to the chemical reactant gas flow into the deposition chamber. Thirdly, terminating the flow of the chemical into the deposition chamber while concurrently substantially matching the pressure downstream from the deposition chamber to the pressure in the deposition chamber to substantially suppress the flow out of the deposition chamber and continuing the chemical dose stage for a specified time without further introduction of chemical flow. This mode of conducting chemical dose during SMFD-ALD process is in-particular advantageous when the pressure downstream to the deposition chamber is preferably increased by flowing gas into a draw gas introduction chamber (DGIC) located in serial fluidic communication downstream from the deposition chamber. 
     Improved ALD performance is also described in terms of apparatus and method to apply a seal-protected slot valve to control the flow restriction properties of the ALD space. According to the invention, an ALD system comprises an improved perimeter slot valve (PSV) in a reactor vessel includes a substrate-transport slot through the reactor vessel wall ( 852 ), a continuous perimeter cavity ( 854 ) within the reactor vessel wall, a continuous sealing poppet ( 856 ′) and an actuator ( 858 ) for moving the sealing poppet between an open position and a closed position. The sealing poppet is moved into the perimeter cavity in the closed position, and out of the perimeter cavity in the open position. The substrate-transport slot is substantially coplanar with a substrate-supporting surface of the substrate holder and the perimeter cavity is substantially coplanar with the substrate-transport slot. The substrate-transport slot defines a substrate-transport channel through the reactor vessel wall to the substrate holder when the sealing poppet is in the open position and separates the substrate-transport slot from the vessel interior when the sealing poppet is in the closed position. The improved seal-protected PSV comprises a fixed upper sealing surface, an upper poppet sealing surface corresponding to the fixed upper sealing surface, an upper peripheral seal, a fixed lower sealing surface, a lower poppet sealing surface corresponding to the fixed lower sealing surface and a lower peripheral seal. According to an important aspect of the invention the upper sealing surfaces, the lower sealing surfaces, and the peripheral seals are configured to seal the vessel interior when the sealing poppet is in the closed position. In this embodiment, the substrate holder is larger than the substrate and the perimeter area of the substrate holder is not covered by the substrate, as a result the sealing poppet creates a substantially peripheral narrow gap between the uncovered perimeter area of the substrate holder and the bottom surface of the poppet when the seal-protected-PSV is in the closed position where the peripheral narrow gap is narrower than the substrate transport slot when the PSV is in the closed position but the radial narrow gap is wider than the substrate transport slot when the PSV is in the open position. In many implementations of the invention, the gap controlling PSV is preferably radially shaped. Most important is that the implementation of SMFD-ALD apparatus design with a DGIC and the SMFD-ALD method enables seal-protected PSV implementation that is suitable and low-maintenance. In a preferred design in accordance with the invention, the bottom surface of the poppet is preferably designed with a substantially down-looking convex shape. Additional improvement taught by the invention utilizes a purge gas that is preferably introduced at substantially low flow between the fixed upper sealing surface and the upper poppet sealing surface to protect the upper sealing surface. In yet another improvement, a PSV also includes a peripheral inflatable seal that is preferably formed between the fixed upper sealing surface and the upper poppet sealing surface and the seal is preferably inflated when the PSV is closed to substantially fill up the downstream from the upper peripheral seal between the fixed upper sealing surface and the upper poppet sealing surface when said inflatable seal is inflated. In another improvement, the inflatable seal is preferably made from a slightly permeable elastomer and the inflatable seal is inflated with high purity inert gas to provide localized purge gas at the area of the inflated seal that is located between the fixed upper sealing surface and the upper poppet sealing surface and is exposed to the process. This localized purge substantially protects the exposed area of the seal from directly contacting the process chemicals. Alternatively, the inflatable seal is preferably made from a perforated elastomer and the inflatable seal is inflated with high purity inert gas to provide localized purge gas at the area of the inflated seal that is located between the fixed upper sealing surface and the upper poppet sealing surface and is exposed to the process. This localized purge substantially protects the exposed area of the seal from directly contacting the process chemicals. 
     The invention further teaches and clearly illustrates in the preferred embodiment description and drawings that the bottom surface of the poppet is advantageously designed with a substantially down-looking convex shape to minimize flow disturbances. 
     Enhanced maintainability of gap-controlling PSV according to another embodiment presented in this invention introduces a purge gas at substantially low flow between the fixed upper sealing surface and the upper poppet sealing surface of the PSV. This purge gas is supplied during processing and protects the inevitable crevice between the poppet and the associated sealing surface from chemical entrapment and the growth of inferior film. 
     Enhanced maintainability of gap-controlling PSV according to yet another embodiment presented in this invention utilizes a radial inflatable seal formed between the fixed upper sealing surface and the upper poppet sealing surface of the gap-controlling PSV. According to this embodiment, the inflatable seal is placed in the gap between the poppet and the corresponding sealing surface, downstream from the upper peripheral radial seal. The seal is preferably inflated when the PSV is closed to substantially close and eliminate the gap downstream from the upper peripheral seal between the fixed upper sealing surface and the upper poppet sealing surface. In a further enhancement, the inflatable seal is preferably made from a slightly permeable polymer, and the inflatable seal is preferably inflated with high purity inert gas. The minimal area of the inflated seal that is preferably located between the fixed upper sealing surface and the upper poppet sealing surface and preferably is exposed to the process and is therefore substantially purged by the flow of the high purity inert gas out of the inflated seal. That flow can be maintained very low to effectively have no distinguishable impact on SMFD-ALD performance. In another variation in accordance with the teaching of this invention, the inflatable seal is preferably made from a perforated polymer and the inflatable seal is preferably inflated with high purity inert gas. The minimal area of the inflated seal that is located between the fixed upper sealing surface and the upper poppet sealing surface and is exposed to the process is substantially purged by the flow of said inert gas out of the inflated seal. 
     The invention also discloses a semi-PSV (SPSV) apparatus for enhancing SMFD-ALD performance. Accordingly, the SPSV includes a substrate-transport slot through the reactor vessel wall, a continuous perimeter cavity within the reactor vessel wall, a continuous sealing poppet, and an actuator for moving the sealing poppet between an open position and a closed position. The sealing poppet is moved into the perimeter cavity in the closed position and out of the perimeter cavity in the open position. The substrate-transport slot is preferably substantially coplanar with a substrate-supporting surface of the substrate holder, and the perimeter cavity is preferably substantially coplanar with the substrate-transport slot. The substrate-transport slot preferably defines a substrate-transport channel through the reactor vessel wall to the substrate holder when the sealing poppet is in the open position. The sealing poppet preferably separates the substrate-transport slot from the vessel interior when the sealing poppet is in the closed position. Specifically, the SPSV further includes a chamber top, a flexible metal bellow seal or a sliding vacuum seal allowing the chamber top to move up and down while maintaining vacuum integrity, a fixed lower sealing surface, a lower poppet sealing surface corresponding to the fixed lower sealing surface, and a lower peripheral seal. The lower sealing surface and the peripheral seal are configured to seal the vessel interior when the sealing poppet is in the closed position. At that position, the poppet essentially defines the top portion of the vessel. In some SPSV designs, the substrate holder is preferably larger than the substrate, and the perimeter area of the substrate holder is not covered by the substrate. The sealing poppet preferably creates a substantially peripheral narrow gap between the uncovered perimeter area of the substrate holder and the bottom surface of the poppet. This peripheral gap is preferably narrower than the substrate transport slot when the SPSV is in the closed position and is preferably equal or wider than the substrate transport slot when the SPSV is in the open position. In important aspects of the invention, the chamber top can preferably include a gas distribution showerhead. The design advantageously enhances SMFD-ALD by providing reduced conductance from the process chamber into the draw chamber or the DGIC. In one advantageous embodiment, the entire ALD manifold is preferably mounted on the moving top of the SPSV and is preferably connected to the process gas and chemical sources with flexible means. In some applications the SPSV preferably has radial symmetry. 
     In one aspect of the invention, an ALD system comprises a perimeter slot valve (PSV) in a reactor vessel including a substrate-transport slot through the reactor vessel wall, a continuous perimeter cavity within the reactor vessel wall, a continuous sealing poppet and an actuator for moving the sealing poppet between an open position and a closed position wherein the sealing poppet is moved into the perimeter cavity in the closed position and out of the perimeter cavity in the open position. The substrate-transport slot is substantially coplanar with a substrate-supporting surface of the substrate holder, the perimeter cavity is substantially coplanar with the substrate-transport slot, the substrate-transport slot defines a substrate-transport channel through the reactor vessel wall to the substrate holder when the sealing poppet is in the open position and separates the substrate-transport slot from the vessel interior when the sealing poppet is in the closed position. The PSV further includes a fixed upper sealing surface, an upper poppet sealing surface corresponding to the fixed upper sealing surface, an upper peripheral seal, a fixed lower sealing surface, a lower poppet sealing surface corresponding to the fixed lower sealing surface and a lower peripheral seal. The upper sealing surfaces, the lower sealing surfaces and the peripheral seals are configured to seal the vessel interior when the sealing poppet is in the closed position. Additionally, a plenum for delivering inert gas into the vessel interior wherein the poppet, the substrate holder and the inert gas delivery plenum are configured to define a peripheral space when the PSV is in the closed position and the inert gas is inserted through the inert gas delivery plenum during chemical dose to substantially reduce the flux of the chemical at the surface of the upper seal and the lower seal. This seal-protected PSV apparatus and method are preferably suitable for PSV with radial symmetry as well as any other symmetry. 
     In another aspect, an SMFD-ALD system is disclosed comprising a reaction vessel defined by reaction vessel wall, a translatable liner, and an actuator to translate the translatable liner between low and high positions. This translatable liner preferably includes a substantially convex surface at the bottom portion that creates a peripheral gap when actuated to the low position creating a substantially symmetric peripheral surface around a substrate holder. The substrate holder is preferably larger than the substrate and a peripheral DGIC space is preferably created between the liner, the substrate holder, and the wall of the reaction vessel when the liner is at the low position. In a further aspect, the liner preferably comprises a gas plenum to deliver inert gas from the upper and the lower peripheral edges of the liner into the peripheral DGIC space and the inert gas is preferably inserted through this gas plenum during chemical dose to substantially reduce the flux of the chemical at the surface of the upper edge and the lower edge of the liner and their respective crevices. The Apparatus disclosed with respect to the translatable liner can be preferably selected to have radial symmetry or other peripheral symmetry. 
     In yet another improvement, this invention discloses an SMFD-ALD system comprising an apparatus for high speed draw control gas delivery into and out-of a DGIC. In a specific aspect, the SMFD-ALD system having high-speed draw control gas delivery apparatus comprises an inlet FRE, a gas reservoir, a shutoff valve, and an outlet FRE all placed in series fluidic communication between an inert gas source and the DGIC. Additionally, a pumping line and a pumping shutoff valve provide serial fluidic communication between the outlet FRE and a vacuum pump. The apparatus is set to shape a time varying inert gas introduction into a parasitic space that is located in serial fluidic communication between the outlet FRE and the DGIC and the time varying inert gas introduction preferably comprises a high-flow leading edge. To facilitate fast reduction of draw-control flow, the parasitic space is preferably connected to the vacuum pump concurrently with the shut-off of the inlet valve. 
     In another aspect, an atomic layer deposition system comprising a deposition chamber, a gas draw chamber, a deposition chamber seal having a seal gap area exposed to the deposition, and a gas purge source connected to the seal gap is disclosed. 
     In another aspect, a method of atomic layer deposition comprises providing an atomic layer depositing apparatus having a deposition chamber and a deposition chamber seal gap, a portion of which seal gap is exposed to the deposition chamber wherein the method includes depositing a thin film in the deposition chamber and purging the seal gap portion exposed to the deposition chamber with purge gas during the deposition is key to maintain the seal and the performance of the ALD system. 
     In another aspect, a disclosed atomic layer deposition apparatus comprises a combination of a deposition chamber housing having a fixed housing portion and a movable portion, and the movable portion is supported on a bellows to maintain vacuum integrity. 
     In another aspect, an atomic layer deposition apparatus comprises a deposition chamber housing having a fixed housing portion and a movable portion, a gap between the fixed housing portion and the movable portion, and an inflatable seal for sealing that gap. In a modified aspect, the seal is preferably perforated and further includes a source of purge gas connected to the gap. In another modification, the apparatus preferably includes a shaped seat against which the seal seats when inflated. In another modification, the apparatus preferably includes a ferrule located interior to the inflatable seal. 
     Improvements are also disclosed for SMFD optimized source design. Apparatuses and methods for generic source design are implemented with commercially available pressure controller for volatile chemicals and with the aid of newly invented apparatuses for lower-volatility chemicals. A chemical vapor source for SMFD-ALD apparatus in accordance with the invention includes a chemical container, a pressure controller in serial fluidic communication downstream from the chemical container, and a source chamber in serial fluidic communication downstream from the pressure controller. The pressure controller is located in serial fluidic communication upstream from the source chamber. The set pressure of the chemical vapor is maintained in the source chamber by the pressure controller. In one preferred design, the chemical source capacity preferably exceeds the capacity loss per ALD cycle by a factor of 10. In another exemplary design, the chemical source capacity preferably exceeds the capacity loss per ALD cycle by a factor of 100. 
     Further, the invention discloses a chemical vapor source for an SMFD-ALD apparatus comprising a liquid delivery system, a vaporizer in serial fluidic communication downstream from that liquid delivery system, and a source chamber in serial fluidic communication downstream from the vaporizer. A set chemical vapor pressure inside the source chamber is maintained by controlling the liquid delivery into the vaporizer. The chemical vapor pressure is measured in the source chamber, and the liquid delivery system is controlled to maintain that pressure at the set point. Therefore, the precision and response of the pressure control depends on the precision of the liquid delivery system and its ability to respond quickly. In one exemplary embodiment, the chemical source capacity preferably exceeds the capacity loss per ALD cycle by a factor of 10. In another exemplary embodiment, the chemical source capacity preferably exceeds the capacity loss per ALD cycle by a factor of 100. 
     Further, an advantageous embodiment for a liquid delivery apparatus is disclosed, exemplified, and illustrated for clarity. Accordingly, a controlled flow of liquid through preferably a proportional valve, a metering valve or a fixed orifice is driven by an expandable pressure chamber. The expansion of that expandable pressure chamber protrudes into a liquid filled chamber. The pressure chamber is separated from the liquid filled chamber with an expandable flexible metallic bellow. When the pressure chamber is inflated, it expands the bellow into the liquid-filled chamber to effectively pressurize the liquid and trigger the flow. The inflation or deflation operations are controlled with a fast solenoid-based valve and the introduction or disposition of air pressure, respectively. Accordingly, liquid flow can be triggered ON or OFF with unprecedented and adequate speed preferably in the range of from 5 msec to 50 msec response time. 
     In another aspect of the invention, a chemical vapor source for SMFD-ALD apparatus is disclosed comprising a chemical source employing a temperature controlled sensor that senses the accumulation of materials on its sensing surface to control the vapor pressure of the chemical within a source space. The walls of the source space are preferably maintained at a temperature that is sufficiently high to prevent condensation of the chemical. Preferably, the sensor is a quartz crystal microbalance (QCM) sensor or a SAW device thickness monitor or any other sensor that can sense the accumulation of material on its sensing surface. Accordingly, the sensor is applied to control the temperature of the chemical to continuously maintain a minimal condensation of the chemical over the sensor. In an aspect of this invented chemical source, the chemical is preferably loaded into a heatable holder, such as a crucible, and the power to heat the crucible is preferably controlled by the sensor. In most applications taught by the invention, the chemical is preferably solid. An important design preferably incorporates chemical delivery into the crucible as a slurry of fine powder and inert liquid preferably liquid with low boiling temperature. The inert liquid preferably does not substantially dissolute the solid chemical and can be vacuum evaporated away from the chemical source to effectively leave a pure and dry solid chemical inside the crucible. In certain applications, the chemical source taught in this invention preferably employs also a combination of pressure gauge and controllable valve to control the total pressure within the source space that exceeds the vapor pressure controlled from the chemical. In that case, the controllable valve preferably delivers inert gas into the source space. A method and apparatus for increasing the versatility of the source is disclosed where the controllable valve preferably delivers an etching gas into the source space and the chemical is preferably generated within the source space. In this aspect, controlling the vapor pressure of the chemical preferably means controlling the temperature of an elemental or compound target and/or the temperature of the etchant to induce sufficient etching and the chemical is essentially the product of said etching. This apparatus and method are especially and preferably useful to produce precursors from the elements Hf, Zr, Ru, RuO 2 , Si, W, Mo, Co, Cu, Al, Fe, Os, OsO 2  and Ta; and the etching gas preferably selected from the list of Cl 2 , Cl 2 /N 2 , Cl 2 /O 2 /O 3 , N 2 /HF, CO, CO/N 2  and their combinations. In another apparatus design, temperature limitations of pressure gauges are preferably overcome by implementing the chemical source using a pressure controlled gas reservoir in series fluidic communication upstream from the source space and a shutoff valve placed in series fluidic communication between the pressure controlled reservoir and the source space where the shutoff valve is preferably used to substantially equalize the total pressure within the source space to the pressure in the pressure controlled reservoir between successive ALD doses. In this case, there is preferably no need to include a heatable pressure gauge within the hot zone of the source and the source useful temperature range is preferably extended beyond the limitation of pressure gauges. The source that is taught in this invention is preferably very useful and adequate for ALD applications when the capacity of the source space is preferably more than 10 times the capacity required for a single ALD dose and even better suited for ALD when the capacity of the source space is preferably more than 50 times the capacity required for a single ALD dose. In conjunction with the chemical source apparatus that is extensively disclosed and exemplified in this invention, a complementary method for substantially controlling the vapor pressure of a chemical within a space employing a temperature controlled sensor to measure the condensation rate of the chemical at the sensor&#39;s temperature is disclosed. The sensor is employed to control the evaporation rate of the chemical to maintain a minimal measurable condensation rate while the sensor&#39;s temperature is selected to appropriately determine the desired vapor pressure of the chemical. The method is preferably extended to cases that preferably require seeding the chemicals into a carrier gas wherein a total pressure higher than the vapor pressure of the chemical is preferably controlled inside the source space and the balance of gas inside the source space preferably comprises an inert gas. The method is even further extended to cases that preferably require seeding the chemicals into a carrier gas wherein a total pressure higher than the vapor pressure of the chemical is controlled inside the source space, and the balance of gas inside the source space preferably comprises an etching gas or an etching gas mixture and the desired chemical is preferably generated by etching an elemental or compound target while the sensor is preferably employed to control the generation rate of the desired chemical to maintain a minimal measurable condensation rate measured on the sensor. The selected generation rate of the desired chemical is preferably controlled by controlling the heating of the target and the sensor&#39;s temperature is selected to appropriately determine the desired vapor pressure of the desired chemical. 
     Thus, the invention also provides a chemical source vapor pressure control system comprising a deposition chamber, a chemical source holder for holding the chemical source, a chemical source heater, a source heater controller, and a deposition accumulation sensor, the heater controller electrically connected to the deposition accumulation sensor to control the heating of the source; the system characterized by: the temperature controlled deposition accumulation sensor located out of line-of-sight with the chemical source; and a sensor temperature control unit for controlling the temperature of the accumulation sensor to a temperature lower than the condensation temperature of the chemical source at the desired vapor pressure. Preferably, the deposition chamber has chamber walls ( 708 ) and further comprising a chamber wall temperature control system for maintaining the walls at a temperature that is sufficiently high to prevent condensation of the chemical source. Preferably, the chemical source vapor pressure control system further includes a pressure gauge, a gas control valve, and a pressure controller connected between the gauge and the valve to control the total pressure within the deposition chamber to a pressure higher than the controlled vapor pressure of the chemical source. Preferably, the chemical source vapor pressure control system includes a source of an etch gas connected to the gas control valve, and the sensor senses an etching product. Preferably, the chemical source is selected from the group consisting of Hf, Zr, Ru, RuO 2 , Si, W, Mo, Co, Cu, Al, Os, OsO 2 , Fe, Ta and combinations thereof; and the etching gas is selected from the group consisting of Cl 2 , Cl 2 /N 2 , Cl 2 /O 2 /O 3 , N 2 /HF, N 2 /ClF 3 , CO, CO/N 2  and combinations thereof. Preferably, the chemical source vapor pressure control system further includes a pressure controlled reservoir ( 780 ); a shutoff valve ( 744 ′) in series fluidic communication between the pressure controlled reservoir and the deposition chamber to substantially equalize the pressure between the deposition chamber and the pressure controlled reservoir between successive ALD doses. Preferably the source is applied for ALD and the capacity of the deposition chamber is 20 times or more larger than the capacity required for a single ALD dose. 
     The invention also provides a method for controlling the vapor pressure of a chemical source within a source space the method comprising: sensing the accumulation of the chemical on a sensing surface; and controlling the temperature of the chemical source depending on the sensed accumulation. Preferably, the temperature of the chemical source is controlled to maintain a minimal measurable condensation rate on the sensing surface. Preferably, the temperature of the sensor is controlled to appropriately determine the desired vapor pressure of the chemical. Preferably, the total pressure in the source space is controlled to be higher than the vapor pressure of the chemical. Preferably, the method includes introducing an etching gas into the source space, and etching an elemental or compound target to produce the chemical. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention may be obtained by reference to the drawings, in which: 
         FIG. 1  depicts a flow diagram of a basic embodiment of a synchronously modulated flow-draw (“SMFD”) ALD system in accordance with the invention; 
         FIGS. 2   a - 2   d  depict in schematic form a comparison between prior art ALD process ( 2   a ), SMFD-ALD process ( 2   b  and  2   c ), and pulse and hold SMFD-ALD process ( 2   d ); 
         FIG. 3  depicts schematically a seal-protected perimeter slot valve at the close position in accordance with the invention; 
         FIG. 4  depicts schematically a seal protected perimeter slot valve at the open position in accordance with the invention; 
         FIG. 5  depicts schematically a gap-controlling perimeter slot valve at the close position in accordance with the invention; 
         FIG. 6  depicts schematically a gap-controlling perimeter slot valve at the open position in accordance with the invention; 
         FIG. 7  highlights the seal area of a gap-controlling perimeter slot valve showing the seal purge line; 
         FIG. 8  depicts schematically a semi-perimeter slot valve at the close position in accordance with the invention; 
         FIG. 9  highlights in schematic form a design for inflatable seal element shown deflated in accordance with the invention; 
         FIG. 10  highlights in schematic form a design for inflatable seal element shown inflated in accordance with the invention; 
         FIG. 11  illustrates the inflatable seal assembly (with reference to  FIG. 10 ) in accordance with the invention; 
         FIG. 12  illustrates the inflatable seal assembly (with reference to  FIG. 10 ) showing the inflation gas line and the connection with the inflatable seal in accordance with the invention; 
         FIG. 13  depicts the same inset as in  FIG. 12  where the seal is shown inflated in accordance with the invention; 
         FIG. 14  depicts a translatable liner in accordance with the invention; 
         FIG. 15  depicts a flow diagram of a basic embodiment of a synchronously modulated flow-draw (“SMFD”) ALD system comprising a sub-manifold for high-speed introduction and removal of draw control flow in accordance with the invention; 
         FIG. 16  depicts schematically a pressure-controlled source for gas and volatile liquid and solid precursors in accordance with the invention; 
         FIG. 17  depicts schematically a pressure-controlled source for non-volatile liquid precursors in accordance with the invention; 
         FIG. 18  depicts schematically a liquid delivery source in accordance with the invention; 
         FIG. 19  depicts schematically a chemical source implementing a sensor that monitors the accumulation of materials such as a QCM for the measurement and control of chemical vapor pressure in accordance with the invention; 
         FIG. 20  depicts schematically a chemical source implementing a QCM for the measurement and control of chemical vapor pressure and an independent total pressure control in accordance with the invention; 
         FIG. 21  depicts schematically a chemical source implementing a QCM for the measurement and control of chemical vapor pressure using an etch target and an independent total pressure control in accordance with the invention; and 
         FIG. 22  depicts schematically a chemical source implementing a QCM for the measurement and control of chemical vapor pressure using an etch target and an independent total pressure replenishing apparatus in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described herein with reference to  FIGS. 1-22 . For the sake of clarity, the same reference numerals are used in several figures to refer to similar or identical components. It should be understood that the structures and systems depicted in schematic form in  FIGS. 1-22  serve explanatory purposes and are not precise depictions of actual structures and systems in accordance with the invention. Furthermore, the embodiments described herein are exemplary and are not intended to limit the scope of the invention, which is defined in the invention summary and in the claims below. 
       FIG. 1  depicts a flow diagram of a basic embodiment of a synchronously modulated flow-draw (“SMFD”) ALD system  600  in accordance with the invention described in U.S. Pat. No. 6,911,092, PCT Application Publication No. WO03/062490, and the improvements that are disclosed in this application. 
     System  600  comprises a pressure-stabilized inert, purge-gas source  602  of a purge gas  604 . Purge gas is supplied through purge-source shut-off valve  102  and purge-source flow restriction element (“FRE”)  103  into gas distribution chamber  104 , which is commonly a conventional shower head. As depicted in  FIG. 1 , purge-source shut-off valve  102  and purge-source FRE  103  provide serial fluidic communication between purge-gas source  602  and gas distribution chamber  104 . In this specification, flow restriction elements (FREs) cause a pressure step-down when gas is flowing through them. A chemical reactant precursor in the form of a pure chemical gas, a vapor from a liquid or solid chemical, or mixtures of vapor or gas chemicals with inert gas is provided at well-controlled pressure at a plurality of chemical-gas sources  105 ,  105 ′. Chemical-gas source  105  is connected in serial fluidic communication with booster chamber  107  through chemical-source-FRE  106 . Booster chamber  107  is connected in serial fluidic communication with gas distribution chamber (showerhead)  104  through chemical-dosage shut-off valve  110  and booster-FRE  109 . As depicted in  FIG. 1 , second chemical-gas source  105 ′ is connected to showerhead  104  with devices corresponding to those described with reference to chemical-gas source  105 . 
     Gas-distribution FRE  113  provides serial fluidic communication between gas distribution chamber  104  and atomic layer deposition chamber (“deposition chamber”)  114 . In a preferred embodiment, in which gas distribution chamber  104  is a showerhead device, gas-distribution FRE  113  is commonly a nozzle array. A nozzle array provides restricted and uniform flow from gas distribution chamber  104  to deposition chamber  114 , which contains a substrate being treated. A substrate supporting chuck with means to control the substrate temperature,  620 , is disposed within deposition chamber  114 . 
     Deposition chamber  114  is connected in serial fluidic communication to a small-volume draw-gas introduction chamber (“DGIC”)  630  through deposition-chamber FRE  115 . Inert draw-gas source  602  is connected in serial fluidic communication to DGIC  630  through draw-gas line  119 , draw-source shut-off valve  120 , and draw-source-FRE  121 . Draw-gas introduction chamber  630  is connected in serial fluidic communication through DGIC-FRE  632  to draw control chamber (“DC”)  116 . A chemical abatement element  634  is disposed inside DC  116 . DC  116  is connected in serial fluidic communication to pump chamber  636  through draw-control outlet  124  and draw-control FRE  117 . A pressure gauge  638  is connected to DC  116 . Pressure gauge  638 , for example, an MKS Baratron® model  628  type, monitors the process through, for example, the average pressure in DC  116 . Similarly, other process monitoring devices (not shown), such as gas analyzers, can be conveniently connected to DC  116 . Low pressure gauge  644 , such as an HPS I-Mag cold-cathode gauge, is attached to pump chamber  636  to monitor chamber pressure during idle time. Turbomolecular pump  640  is connected to pump chamber  636  through a pumping gate-valve  642  to facilitate high vacuum during idle time and high-throughput flow during ALD operation. For example, a pump selected from the BOC-Edwards STPA series is suitable. Good performance for ALD deposition on 200 mm silicon wafers was obtained using an STPA 1303C pump. Turbomolecular pump  640  is evacuated using backing pump  646 . For example, a BOC Edwards QDP40 or equivalent serves well as backing pump  646 . In other embodiments in accordance with the invention, higher pumping-speed pump arrangements, such as the QMB series from BOC Edwards, facilitate remote location placement of dry pumps, as known in the art. 
     In certain preferred embodiments, reactive gas is added to DC  116  to enhance chemical abatement. Accordingly, system  600  comprises an ozone-supply manifold. Oxygen, oxygen-argon, or oxygen-nitrogen mixtures are supplied from gas cylinder  650 . A mass flow controller  652  controls the flow of gas into a commercially available ozone generator  654 . For example, the MKS Astex AX8407 series ozone generators perform well in SMFD system  600 . The output from ozone generator  654  is monitored by ozone monitor  656 , allowing feedback-control stabilization of ozone concentrations. Pressure controller  658 , for example, an MKS  640 A type, maintains a selected constant pressure inside ozone generator  654 . For the purpose of pulsing ozone into DC  116  while maintaining controlled flow and pressure that are necessary for correct operation of ozone generator  654 , an ozone reservoir  660  comprises a volume selected to suppress the impact of ozone pulsing on the pressure inside ozone generator  654 . This allows pulsing of reactive ozone into DC  116 , while maintaining a desired flow and pressure in ozone generator  654 . Pressure controller  662  controls the pressure in ozone reservoir  660 . Ozone degradation is minimized in system manifold  600  by maintaining the ozone supply manifold at substantially room temperature and by minimizing the stagnant volume between ozone generator  654  and DC  116 . For example, the stagnant volume is described schematically in  FIG. 1  by the dead-leg between valve  664  and junction  668 . Ozone is fed to ozone shut-off valve  664  and ozone-source FRE  666  through the inner tubing of a double-wall line and fed to the inlet of pressure controller  662  by the return flow between the inner and the outer tubing. In this manner, the impact of ozone depletion in the stagnant space is minimized by reducing the dead-leg between valve  664  and junction  668  to less than 1 cc. Preferably, an ozone-eliminating catalytic converter  670  is disposed at the outlet of pump  642  to suppress ozone emission to the ambient. 
     In a preferred embodiment, the functionality of chemical-dosage shut-off valves  110 ,  110 ′ was integrated into a multiple-port chemical introduction valve manifold comprising both  110  and  110 ′. Fast pneumatic valves with millisecond response time, described in a separate patent by the inventor of this invention were mainly utilized successfully for that purpose. 
     During typical ALD operation, apparatus  600  is switched essentially between two static modes, a purge mode (“purge”) and a chemical-dosage mode (“dose”). Representative valve-settings of the two basic modes of operation are presented in Table 1. More teaching about the SMFD ALD apparatus and method is given in U.S. Pat. No. 6,911,092 and PCT Application Publication No. WO03/062490. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Mode 
                 Valve 102 
                 Valve 120 
                 Valve 110 
               
               
                   
                   
               
             
            
               
                   
                 Purge 
                 OPEN 
                 CLOSED 
                 CLOSED 
               
               
                   
                 Chemical dosage 
                 CLOSED 
                 OPEN 
                 OPEN 
               
               
                   
                   
               
            
           
         
       
     
       FIGS. 2   a - 2   d  present a schematic comparison between prior art flow versus time chart  300  presented in  FIG. 2   a  and the inlet flow into an SMFD showerhead  104 , chart  320  presented in  FIG. 2   b . Since typical SMFD timing is more than 5 times shorter, the time scale of the SMFD charts is divided by a factor of 5.  FIG. 2   c  presents chart  340  which represents the complementary flow versus time into the DGIC  630  in synchronization with the inlet flow depicted in chart  320  ( FIG. 2   b ). An ALD cycle comprised of first chemical dose  302 , first purge  304 , second chemical dose  306 , and second purge  308  is conventionally carried under substantially constant flow conditions as illustrated in chart  300 . In contrast, the inlet sequence  320  of first chemical dose  322 , first purge  324 , second chemical dose  326 , and second purge  328  is carried under substantially modulated flow conditions. Complementary draw flow presented in chart  340  maintains the pressure during chemical dose steps with draw flow  342  and  344  during chemical dose steps  322  and  326 , respectively. Note the transient stages  321  and  325  at the leading edge of chemical dose  322  and  326 , respectively. These booster high-flow leading edge transients are further taught in U.S. Pat. No. 6,911,092 and PCT Application Publication No. WO03/062490. 
     A further improvement in chemical utilization splits the chemical dose steps into pulse and hold in accordance with Table 2. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Mode 
                 Valve 102 
                 Valve 120 
                 Valve 110 
               
               
                   
                   
               
             
            
               
                   
                 Purge 
                 OPEN 
                 CLOSED 
                 CLOSED 
               
               
                   
                 Chemical dose PULSE 
                 CLOSED 
                 OPEN 
                 OPEN 
               
               
                   
                 Chemical dose HOLD 
                 CLOSED 
                 OPEN 
                 CLOSED 
               
               
                   
                   
               
            
           
         
       
     
     This improvement is depicted in  FIG. 2D . The flow versus time is illustrated in chart  360 . Accordingly, the first chemical dose includes transient  361 , low flow steady-state  362 , and no flow period  363 . Similarly, the second chemical dose includes transient  365 , low flow steady-state  366 , and no flow  367 . When valve  110  shuts off during the dose, the flow into the deposition chamber ceases. The deposition chamber becomes a dead-leg and the pressure in deposition chamber  114  transients down slightly to match the pressure in DGIC  630 . If chemical depletion is not prominent, dose time is extended at no additional increase in chemical utilization. This improved mode of chemical dose can be utilized to improve the quality of ALD films by extending chemical reaction time to facilitate further completion of these reactions. The global rate of the typically first order ALD reactions is proportional to the flux of chemicals and concentration of unreacted sites. Naturally, as the reaction propagates, the number of reactive sites decreases and accordingly the global reaction rate decreases. In many ALD processes, the inevitable residual concentration of reactive sites that is not reacted at the end of the chemical dose step contributes to the inclusion of impurities in the film. In particular, embedded OH groups are detrimental to insulating properties of dielectric ALD films. Therefore, in some ALD processes, film quality may require extended chemical dose exposures. The pulse and hold mode given in Table 2 and illustrated in chart  360  advantageously extends the actual dose exposure while avoiding the penalty of increasing chemical utilization. For example, trimethylaluminum (TMA) dose of 50 msec was utilized with only 10 msec to 20 msec of pulse and complementary 30 msec to 40 msec of hold on our SMFD system. The pulse and hold mode further improves chemical utilization efficiencies by eliminating any pressure gradients in deposition chamber  114  during the HOLD time. This decoupling between the exposure (flux multiplied by dose time) and the utilization of chemical is a key advantage of pulse-and-hold SMFD. 
     The pulse-and-hold mode enables efficient chemical utilization during low temperature ALD. ALD reactions are thermally activated. At low temperatures, ALD reactions are slow and inefficient. To minimize dose times, ALD precursors are dosed at maximized pressure and 100% concentration. Pulse and hold SMFD mode is advantageously implemented to suppress the loss of chemical during dose. A pulse and hold sequence was implemented for TMA dose during SMFD-ALD of Al 2 O 3  at 100° C. A fully saturated process step required only 20 msec of pulse and 30 msec of dose (total of 50 msec dose time). Material utilization was greater than 5%, which is extremely good for such low temperature process that was executed with such short dose time. 
     A useful range for dose was attempted successfully between 5 msec to 120 msec, and a useful range of hold was tested from a minimum of 5 msec up to 200 msec. Preferably, dose is carried with a 5 msec to 50 msec duration, and hold is carried with a 20 msec to 100 msec duration. Mostly a preferred range of 5 msec to 25 msec for dose and 15 msec to 35 msec for hold is recommended. 
       FIGS. 3 and 4  depict in schematic form a cross-section of a preferred ALD reactor vessel  800 . As shown in  FIG. 3 , reactor vessel  800  comprises a reactor vessel wall  802 , a reactor vessel top  804 , and a vessel-bottom  806 , which define a vessel interior  808 . Reactor vessel  800  includes gas distribution chamber (showerhead)  201 . A showerhead inlet  809  at top  804  serves as an inlet for chemical reactant gases and purge gases into showerhead  201 . Nozzle array flow restricting element (FRE)  202  separates the bottom of gas distribution chamber  201  from ALD deposition chamber (process chamber)  203 . A substrate  204  is supported on heated wafer chuck (substrate holder)  205 , made from a thermally conducting metal (e.g., W, Mo, Al, Ni) or other materials commonly used in the art for hot susceptors and chucks. Wafer chuck  205  includes a wafer lift-pin mechanism  810 . Wafer transport is accomplished with aid of lift pins  812  (only one out of three pins shown), as known in the art. Wafer lift pins  812  are actuated to lift wafer substrate  204  above the top surface of chuck  205  using actuator  814  and levitation arm  816 . Deposition chamber  203  is confined by deposition-chamber FRE  206  representing a typically peripheral passage between top  804  and chuck  805 . A draw-gas introduction chamber (“DGIC”)  820  is located downstream from deposition chamber  203 , between FRE  206  and DGIC-FRE  822 . A draw control chamber (“DC”)  208  is located downstream from DGIC, and is confined by DGIC-FRE  822  and draw-control FRE baffle  209 . Chemical-abatement element  824  is disposed inside DC  208 . Spacer  826  provides direct thermal contact of chemical-abatement element  824  and draw-control FRE baffle  209  with heated wafer chuck  205 . 
     Draw-gas inlet  830  provides serial fluidic communication between a draw-gas manifold (not shown) and a draw gas plenum  832 . One skilled in the art can implement draw gas plenum  832  in many different configurations, and the embodiment shown in  FIGS. 3 and 4  is a non-exclusive example. As depicted in  FIG. 3 , draw-gas inlet  830  is in fluidic communication with radial plenum space  832 , which further communicates with DGIC  820  through a radial array of nozzles (not shown), which are appropriately spaced and designed to unify the radial flow distribution of gas into DGIC  820  and direct draw gas into the upstream portion of DGIC  820 . Those who are skilled in the art can appreciate the necessity to adequately unify the flow of draw gas and reactive abatement gas to conform to the symmetry of the deposition system. For example, the radial symmetry of the system is depicted in  FIGS. 3 and 4 . Indeed, a draw control gas, introduced with a substantially non-uniform radial distribution, impacted the radial distribution of dosed chemicals as observed when ALD was tested with one of the dose steps kept under saturation conditions. While saturation properties of ALD reaction steps can overcome this effect, longer chemical doses are dictated that, in turn, extend the ALD cycle time and in many cases reducing the chemical utilization efficiency. As explained in U.S. Pat. No. 6,911,092, and PCT Application Publication No. WO03/062490, and further below, the draw control gas, the DGIC, and the SMFD method are crucial and instrumental in enabling the implementation of perimeter slit-valve to improve performance and reduce the size of the ALD chamber. In that respect, the plenum and the DGIC protect the seals of the PSV and their respective crevices from a substantially damaging contact with the process chemicals. 
     Optionally, reactive gas is delivered from a reactive gas manifold (not shown) through line  840  into reactive-gas plenum  842 . Reactive-gas plenum  842  serves to shape a uniform radial flow distribution of reactive abatement gas into draw chamber  208 . For example, the reactive gas is delivered into a radial channel that communicates with draw chamber  208  through a plurality of horizontal nozzles that are appropriately spaced and designed. One skilled in the art can appreciate that reactive gas plenum system  842  can be implemented in many different configurations in accordance with the invention. 
     During ALD processing, purge gas during a purge stage and chemical reactant gas during a dosage stage flow along a process-gas flow-path through reactor vessel interior  808  in a downstream direction from showerhead inlet  809  through showerhead  201 , deposition chamber  203 , DGIC  820 , and DC  208 , in that order, and out of reactor vessel  800  through vacuum port  210 . Similarly, draw gas introduced into DGIC  820  flows in a downstream direction from DGIC  820  into DC  208  and then exits through vacuum port  210 . The terms “downstream” and “upstream” are used herein in their usual sense. It is a feature of embodiments in accordance with the invention that backflow of gases, that is, the flow of gases in an “upstream” direction, never occurs, as taught in U.S. Pat. No. 6,911,092 and PCT Application Publication No. WO03/062490. The term “upstream” is used in this specification, however, to designate the relative locations of components and parts of a system. 
     Reactor vessel  800  further includes a perimeter slot valve (“PSV”)  850 . As depicted in  FIGS. 3 and 4 , PSV  850  comprises a substrate-transport slot  852  through reactor vessel wall  802 , a continuous perimeter cavity  854  ( FIG. 4 ) within reactor vessel wall  802 , a continuous sealing poppet  856 , and an actuator  858  for moving sealing poppet  856  between an open position ( FIG. 4 ) and a closed position ( FIG. 3 ). Sealing poppet  856  is moved into perimeter cavity  854  in the closed position ( FIG. 3 ), and sealing poppet  856  is moved out of perimeter cavity  854  in the open position ( FIG. 4 ). Substrate-transport slot  852  is substantially coplanar with the substrate-supporting surface of substrate holder  205 . Perimeter cavity  854  is substantially coplanar with substrate-transport slot  852 . Substrate-transport slot  852  defines a substrate-transport channel through reactor vessel wall  802  to substrate holder  205  when sealing poppet  856  is in open position ( FIG. 4 ), and sealing poppet  856  separates substrate-transport slot  852  from vessel interior  808  when sealing poppet  856  is in its closed position ( FIG. 3 ). 
     Reactor vessel wall  802  defines a vessel perimeter within the reactor vessel wall, and sealing poppet  856  conforms to the vessel perimeter when sealing poppet  856  is in its closed position ( FIG. 3 ). As depicted in  FIGS. 3 and 4 , reactor vessel wall  802  comprises a substantially radially symmetric shape, and sealing poppet  856  comprises a substantially radially symmetric shape in the case wherein the chamber symmetry is substantially radial. It is understood that other embodiments of reactor vessel  800  and PSV  850  in accordance with the invention could have other geometric shapes. As depicted in  FIG. 3 , sealing poppet  856  in its closed position forms an inner sealing wall  862  of the process-gas flow-path in vessel interior  808 . Inner sealing wall  862  comprises a radially symmetrical shape, which promotes a radially symmetric flow of gasses along the process-gas flow-path and, thereby, enhances uniform deposition and reduces formation of solid deposits. In the particular embodiment of reactor vessel  800  as depicted in  FIG. 3 , a portion of inner sealing wall  862  defines a portion of DGIC  820 . As depicted in  FIG. 4 , PSV  850  comprises a fixed upper sealing surface  870 , an upper poppet sealing surface  872  corresponding to fixed upper sealing surface  870 , an upper peripheral seal  873 , a fixed lower sealing surface  874 , a lower poppet sealing surface  876  corresponding to fixed lower sealing surface  874 , and a lower peripheral seal  877 . Upper sealing surfaces  870 ,  872 , lower sealing surfaces  874 ,  876 , and peripheral seals  873 ,  877  are configured to seal the vessel interior when sealing poppet  856  is in its closed position ( FIG. 3 ). 
     As depicted in  FIG. 4 , upper peripheral seal  873  and lower peripheral seal  877  are assembled on poppet sealing surfaces  872 ,  876 , respectively. Also, seals  873 ,  877  are configured as o-ring seals. It is clear that different types of seals, for example, flat gasket seals, are useful, and that seals  873 ,  877  can be assembled on fixed sealing surfaces  870 ,  874 , instead of on poppet sealing surfaces  872 ,  876 . Suitable materials for seals  873 ,  877  include elastomer materials made from Viton, Kalrez, Chemraz, or equivalents. One skilled in the art is capable of implementing perimeter slot valve  850  in many different configurations. 
     Substrate-transport slot  852  and the associated wafer transport system communicated through slot  852  are completely isolated from the ALD process system in reactor vessel interior  808  when PSV  850  is closed. 
     The implementation of the preferred embodiment has revealed that indeed the high flow of inert gas into the leading edge of the DFIC during chemical dose was sufficient to provide exceptional protection against possible film buildup in radial crevices  882  and  884  that are formed between  804  and  856  and between  856  and  802 , respectively. Accordingly, seal-protected PSV was implemented with no adverse impact on maintenance cycle or performance. These adverse effects are typical and practical limitations in the case of a simple ring-shaped slit-valve implementation as taught in U.S. Pat. No. 6,347,919. Additional improvements that enable the implementation of perimeter slit-valve in ALD apparatuses and other processing chamber are taught in an additional patent by the inventor of this invention. 
     The PSV can be further utilized to reduce the conductance of FRE  115  between process chamber  114  and DGIC  630  ( FIG. 1 ). Smaller FRE  115  conductance increases the pressure gradient between process chamber  114  and DGIC  630  with several fold advantages. First, a better suppression of backflow is established. Second, at any given flow, the pressure gradient across process chamber  114  is reduced. Finally, the DGIC is better defined and the requirements for draw flow radial uniformity are relaxed. However, in the embodiment of  FIGS. 3 and 4 , the range for narrowing the gap between  804  and  205  which defines the conductance of FRE  115  is limited by the need to provide a convenient path for wafer transport. However, in the embodiment presented in  FIGS. 5 and 6 , the constraint of the wafer-loading path is removed with the implementation of a gap-controlling PSV. As illustrated in  FIG. 5 , the gap-controlling PSV implements a convex lower surface  880  on the bottom of a wider poppet  856 ′ to narrow the gap  206  between  880  and chuck  205 . The resulting conductance of gap  206  can be as low as necessary since, as shown in  FIG. 6 , when the PSV is opened to facilitate wafer transport, gap-controlling surface  880  is raised and, therefore, does not interfere with the transport path. Gap-shaping portion  880  of PSV poppet  856 ′ is preferably shaped with a down-looking substantially convex smooth continuation of part  804  to minimize flow disturbance. 
     To facilitate the gap-controlling PSV, the inner sealing gaskets and gasket grooves  872 ′ and 873′ are relocated as illustrated in  FIG. 6 . Likewise, top sealing surface  870 ′ is relocated. 
     In the PSV embodiment displayed in  FIG. 3 , crevices  882  and  884  next to sealing gaskets are effectively shielded from the ALD precursors by the high flow of inert gas in the DGIC. As taught above this is the crucial feature that enables the implementation of, otherwise practically useless, PSV for the SMFD ALD apparatus. However, in the gap-controlling PSV embodiment ( FIG. 5 ), the gap of the inner seal is located inside process chamber  114  and, therefore, is no longer protected. Accordingly, entrapment of ALD precursors can adversely impact the memory of the ALD chamber and can lead to fast deterioration of the inner seal and respective crevice if growth of inferior films in the gap is not suppressed. To overcome this problem, the gap  882  must be purged with a slow flow of inert gas during chemical dose.  FIG. 7  illustrates schematically the seal area of the PSV. Only the right side of a cross-sectional view is shown. Gap  882  between poppet  856 ′ and top  804 ′ is purged through a delivery line  886  that is machined into the body of part  804 ′. 
     In another embodiment illustrated in  FIG. 8 , the inner seal of the PSV is completely eliminated and poppet  856 ″ forms a solid assembly with top part  804 ″. Bellow  888  allows the entire assembly to elevate when the Semi PSV (SPSV) is moved to the OPEN position. In this case, purge gas connection line  612  and the connections of chemical sources  105  and  105 ′ ( FIG. 1 ) are made flexible to accommodate an ˜12 mm of vertical motion. Accordingly, flexible hoses, bellows, or high purity Teflon line sections are implemented. It is appreciated that other means for retaining vacuum integrity while providing motion for 856″+804″ assembly can substitute for the bellow seal shown in  FIG. 8  without deviating from the scope of this invention. 
     Another embodiment that is well-suited to eliminate the pitfalls of crevice  882  associated with the inner PSV seals is presented in  FIGS. 9 ,  10 ,  11 ,  12 , and  13 . In this embodiment, crevice  882  is protected by an inflated elastomer seal. The elastomer seal is made, for example, from suitable materials such as Viton, Kalrez, Chemraz, or equivalent and is mounted inside ledge  890  located under seal surface  870 ″. When the PSV is at the upper position (PSV OPEN), elastomer  892  is not inflated as shown in  FIG. 9 . When the PSV is at the lower position (PSV SHUT), elastomer  892  is inflated by applying inert gas or air pressure through conduit  894 . As a result, inflated elastomer  892  creates a seal against an appropriately shaped surface on poppet  856 ″′. For example,  FIG. 9  depicts a concave shaped portion  889  of  856 ″′ that accommodates the curved shape of inflated seal  892 . Following this inflation, crevice  882  is eliminated and only a small portion  896  of inflated seal  892  is exposed to the process ( FIGS. 10 and 13 ). In one preferred embodiment of this invention, the inflated seal is made from a slightly permeable elastomer. Inflation with inert gas results in a slow flow of inert gas through the elastomer at exposed area  896 . Accordingly, this inert gas flow suppresses the growth of films on exposed area  896  during process. In another alternative embodiment, the elastomer is appropriately perforated at the  896  area to provide a path for inert gas flow and protection to area  896  from process chemicals. 
     Inflated elastomer seal  892  can be implemented in many different designs according to this invention. For example,  FIGS. 11 and 12  illustrate a specific preferred design. Part  804 ″ is split into an inner portion  898  and an outer portion  900 . Appropriately-shaped elastomer seal  892  is folded and pressed between  898  and  900  and sealed into a substantially triangular-shaped tube by the pressure of upper and lower sealing ledges  902  and  904 , respectively ( FIG. 11 ). Inflation path  906  is machined into one or both of inner and outer parts  898  and  900  as depicted in  FIG. 12 . Against that path, elastomer  892  is appropriately shaped to conform around a metallic ferrule  908 . The pressure of sealing ledges  910  and  912  seals the elastomer over ferrule  908  and in communication with inflation channel  906 . Accordingly, an inflation/deflation path  914  is created.  FIG. 13  displays a larger view of seal  892  after inflation. 
     In another embodiment, SMFD ALD apparatus is implemented with a sliding liner that replaces the PSV in providing symmetrical and dead-leg free ALD processing space.  FIG. 14  illustrates a preferred embodiment wherein a radially shaped sliding liner is translatable to determine both the draw control plenum and the DGIC. The sliding liner  940  is depicted in the process position. In process position, the sliding liner creates well-defined and well-restricted flow paths  930  and  932  that are used to deliver the inert gas draw flow into DGIC  820  during chemical dose. During process, the entire volume  934  behind liner  940  is pressurized with inert gas through inlet  830 ′. This volume includes the slit-valve related cavity  922 . Accordingly, a well-optimized draw flow plenum and FRE gap  206 ′ are established. In addition, a slit-valve cavity  922  created by the interface with planar slit-valve  920  has no adverse impact on the ALD process and is protected from the growth of inferior deposits. When the chamber is set to facilitate wafer transport, the sliding liner is removed from the loading path  922  using actuator  858 ′ while bellows  936  preserve the vacuum integrity of space  934 . 
     The preferred SMFD ALD method implements liner  940  ( FIG. 14 ) in conjunction with a sub-manifold that enables high-speed draw flow control in spite of a substantial volume related to space  934 . For example, sub-manifold  960  depicted in  FIG. 15  is used to vary the pressure within  934  quickly to facilitate fast variation of DC flow into DGIC  630  ( 820  in  FIG. 14 ). Preferably, the flow restriction of FRE  930  and FRE  932  are selected to direct the majority of the draw control flow through FRE  930 . When valve  120 ′ is shut, the booster volume  952  is pressurized to substantially reach the pressure at point  602 . FRE  950  is substantially less restrictive than FRE  121 ′. Accordingly, when valve  120 ′ is opened the flow into space  934  resembles a high flow determined by the flow through FRE  950  that levels off into a substantially lower flow that is mainly dictated by FRE  121 ′. The high flow leading edge facilitates quick pressure increase within space  934  to initiate fast turn “on” of draw control gas. To facilitate fast turn “OFF” of draw control gas, the combination of shutting valve  120 ′ off and opening evacuation valve  956  is implemented to quickly reduce the pressure within space  934 . In the preferred method, the evacuation valve  956  is maintained open to reduce the pressure within space  934  down to a pressure that is still slightly higher than the process pressure to facilitate minimal flow of inert gas through both FRE  930  and FRE  932  during purge. The design of liner  940  must comply with basic SMFD design requiring that the pressure in the DGIC  630  will not be able to exceed the pressure in the process chamber  114 . This requirement sets an upper limitation on the pressure rise rate within DGIC  630  not to exceed the typical 3-4 msec residence time within ALD chamber  114 . 
     SMFD advantageously lends itself to some simplified chemical source apparatuses and methods. In particular, the ability of SMFD to dose vaporized liquid and solid chemicals without a carrier gas is compatible with a simplified pressure-controlled chamber source where the vapor pressure of the chemical can be accurately controlled as described in the exemplary embodiments below. Accordingly, the difficulty to control the partial pressure from chemical precursors in the flow of carrier gas is circumvented. Several different SMFD sources are depicted in  FIGS. 16 ,  17 ,  18 ,  19 ,  20 ,  21  and  22 . Source chamber volume is chosen to improve pressure-stability and to compensate for the slow response of the pressure control device or method that is typically limited within the 1-10 seconds range. Accordingly, it is advantageous to ensure that source chamber capacity (i.e., in liter×Torr) is substantially larger than the material delivered per cycle. For example, source capacity that is 20 times to 100 times larger than the capacity loss per dose lends itself to a minimized pressure ripple in the source chamber, i.e.,  316  in  FIG. 18 , within 1% to 5%, which is tested to have indistinguishable impact on the consistency and length of the chemical dose. Pressure control devices cannot follow the speed of SMFD dose cycles. Rather, as detailed herein, these pressure controlling devices or methods are preferably set to control the average pressure in the source chamber while the capacity within the source is set to smooth out significant ripples. Therefore, the volume of the source is chosen to limit pressure fluctuation, as necessary. 
     Several pressure control methods are described in  FIGS. 16-22 . In  FIG. 16 , a relatively high vapor pressure from liquid or solid chemical  306  is controlled by a commercially available pressure controller  312  such as the MKS Instruments  640 A series which is limited to operation temperature in the range from 0° C. to 50° C. The chemical is located at a separate container  302  and is heated or cooled per temperature control element  304  to provide a pressure, P chem , that is larger than the pressure that is needed in source  316 . This pressure is fed into the inlet of pressure controller  312  that controls the pressure inside source chamber  316 . A shut-off valve  308  is preferably placed between chemical container  302  and pressure controller  312  to terminate chemical supply through conduit  310  when processing is complete. Correct choice of appropriate pressure controller and chemical temperature ensures stable and consistent chemical delivery. In particular, the choice of pressure controller conductance must be suitable for the necessary flow under given pressure conditions as known in the art and described, for example, in the user manual of the  640 A pressure controllers from MKS Instruments. The temperature of source  300 , including gas line  310  interconnecting the chemical container with the pressure controller, pressure controller  312 , the gas line  314  interconnecting the pressure controller and source chamber  316 , and gas line  318  interconnecting the source chamber with the SMFD-ALD manifold at source points  105  and  105 ′ ( FIG. 1 ) must be maintained at a temperature adequately high to prevent condensation of the precursor chemicals, as known in the art. Source evacuation is accomplished through utility valve  320 , conduit  322 , and vacuum pump  324 . 
     In another embodiment  400 , depicted in  FIG. 17 , consistent and controlled pressure from relatively non-volatile liquid chemicals is achieved by applying liquid delivery techniques to deliver the precursor with precision into a vaporizing chamber. Vaporization chamber  406  is connected to source chamber  402  through heated gas line  408 . The pressure is monitored at the source chamber using a conventional pressure gauge  404  such as the model  628 B or the model  631 A Baratron manufactured by MKS Instruments, which are suitable to reliably measure the pressure of chemicals and can be maintained at temperatures of 100° C. and 200° C., respectively, to prevent condensation of non-volatile chemicals. Vaporized precursor is delivered to chemical source point  105  through conduit  412 . The entire assembly downstream from vaporizer  406  is controlled at a temperature suitable to prevent condensation of the chemical. In certain embodiments, the temperature of vaporizer  406  is controlled separately and independently to improve vaporization efficiency, speed, and control. Valve  416  is utilized to evacuate the source chamber through conduit  418  and vacuum pump  420 . 
     Liquid delivery control system  400  does not need to accommodate the ALD dose response, but rather to be able to sustain a consistent delivery over a longer time scale. However, most commercially available liquid delivery systems are not suitable to deliver such small quantities as required for ALD practice that are in the order of 10 −4  cc/cycle. In the case of SMFD with ˜2 cycles/sec, the liquid delivery system must be able to precisely control flow on the order of 0.012 cc/min. This minute chemical flow is in the low range of for example, top-of-the-line DLI-25C system manufactured by MKS Instruments (low limit of 0.006 cc/min). In addition, to maintain the volume of the source chamber conveniently small, the liquid delivery system must accommodate a relatively fast start/stop operation, preferably on the order of a cycle time which is difficult to achieve with commercially available technology. An embodiment that accomplishes consistent delivery of small liquid flow with fast response is described with respect to the schematic illustration given in  FIG. 18 . 
     Liquid delivery system  450  implements container  454  that is filled with liquid precursor  452  from chemical line  472  through inlet valve  470 . Line  472  is connected to a liquid filling line (not shown) that draws liquid precursor from a chemical reservoir such as the EpiFill® system from Epichem, Inc. A variable air chamber  456  completes the makeup of container  454  with a flexible bellow  458 . The liquid can be pressurized by introducing air from pneumatic line  464  through valve  462  into air chamber  456  to force chamber  456  to expand downwards and pressurize the liquid. The liquid can be depressurized by evacuating the pressurized air out of chamber  456  through valve  466  into line  468 . Accordingly, the liquid can be pressurized and depressurized within 5 msec to 50 msec with standard solenoid valves. Variable orifice  460 , for example, a proportional valve, is used to set the flow of liquid towards outlet  410  when the liquid is pressurized. When metallic bellow  458  approaches maximum extension, container  454  is automatically refilled. For example, bellow  458  approaches the bottom of container  454  where a magnetic proximity sensor is mounted. A magnet inside chamber  456  is sensed at proximity and the system will refill container  454  within the next idle time. The proximity sensor is designed to sense the need for refill when system  450  is still capable of delivering enough chemical for an entire interval between idle times. Alternatively, two liquid delivery systems  450  may be connected to a single source  400  and alternately serve and refill. Refilling is accomplished by depressurizing chamber  456  and pushing liquid through valve  470  to retract bellow  458  and refill container  454 . 
     Solid chemicals present several source design challenges. In particular, the inconsistency of evaporation rate from solid chemicals due to fluctuation in the area of the solid material makes seeding vapors from solid source into carrier gas expensive and unreliable. In addition, thermal contact of solid chemicals usually in a shape of fine-grain powders is typically poor leading to significantly inefficient and slow sublimation rates. Accordingly, it is very difficult to maintain consistent and non-depleted supply of vaporized molecular precursors from solid sources. This difficulty applies for both pure vapor form or as partial pressure within a carrier gas. 
     An embodiment  700  disclosing a vapor source from solid chemicals is described here with reference to  FIG. 19 . The source implements a technique to monitor the condensation rate of condensable materials for indirectly evaluating the vapor pressure of these condensable materials. A temperature controlled sensor  710  senses the accumulation of materials on its sensing surface  711 . Sensor  710  is preferably a Quartz crystal Microbalance (QCM), a Surface Acoustic Wave (SAW) device sensor, or other thickness monitoring devices or techniques. Sensor  710  continuously indirectly probes the vapor pressure of the molecular precursor in the following manner. The molecular precursor is sublimated using resistive heating or other suitable means to maintain a minimal growth rate of condensed film of molecular precursor on the material accumulation sensor, e.g. a QCM in the preferred embodiment. Hereinafter, a QCM is used as the exemplary sensor, though it should be understood that other sensors can be used. The QCM sensor is prevented from having a line-of-sight with the sublimation source and therefore the growth of condensed film on it represents condensation of excessive vapor pressure. The temperature of the entire source,  708 , with the exception of the QCM sensor is maintained sufficiently high to prevent condensation at the desired precursor vapor pressure. Commercially available QCM sensor heads are capable to control the deposition or condensation of films on their exposed area with a rate typically better than 2% of a monolayer per second. To facilitate the desired vapor pressure, the temperature of the QCM sensor is maintained by a sensor temperature control system  713  at a pre-selected temperature. The pre-selected temperature for the sensor is several degrees lower than the condensation temperature of the molecular precursor at the desired controlled vapor pressure. Accordingly, by controlling the sublimation source to sustain a minimal condensation rate on the QCM sensor the molecular precursor is maintained at a desired, substantially-controlled, vapor pressure. Commercially available deposition controllers and sublimation sources are proven to be able to reach well-controlled deposition rates within several seconds. Accordingly, source  700  is capable to go from a standby mode wherein the vapor pressure of the molecular precursor is insignificant to a process mode wherein the desired precursor vapor pressure is maintained inside the source within several seconds. Source  700  is also capable to replenish the molecular precursor in the source on a time-scale of several seconds. Accordingly, a source with a well-designed capacity that accommodates only minor pressure drop during ALD dose, as described above, is well suited for high-productivity ALD applications. With only seconds required to cross from idle source with negligible vapor pressure to “active” source with appropriately controlled vapor pressure, the source is practically set to idle in between successful wafer processing. During a typical semiconductor wafer processing of 3 minutes or less, the condensation on the QCM will accumulate on several monolayers. This negligible thickness will be sublimated away from the QCM surface during idle mode. 
     Source chamber  702  is connected to source point  704  (equivalent to  105  and  105 ′ in  FIG. 1 ) through an appropriately heated conduit (not shown). The volume of source chamber  702  is chosen to reduce pressure fluctuations and to accommodate the capabilities of commercially available deposition controllers and sensors as described above. The source is equipped with a temperature controlled QCM sensor  710  having a sensing area  711 . For example, the BSH-150 Bakeable sensor head available from Maxtek, Inc. is proven to work reliably in the temperature range from 30° C.-300° C. The temperature of sensor  710  is tightly controlled using a combination of resistive heating and air-flow cooling. Preferably, the temperature of the sensor  710  is controlled within better than ±0.1° C. to suppress sensor fluctuations and drift. Sensor  710  is preferably modified to seal access to the back side of the quartz crystal microbalance using, for example, Viton cement or other complying adhesives with high temperature compatibility as well as commercially available high temperature elastomer seals such as Kalrez O-rings or equivalents. 
     Sensor  710  is monitored by deposition controller  716  such as commercially available MDC360C from Maxtek, Inc. or the IC/5 controller available from Inficon. The QCM is capable of measuring thickness with better than 0.1 {hacek over (A)} resolution that is equivalent to better than 4% of a monolayer of most materials Likewise, commercially available deposition controllers are capable of controlling the power to evaporation sources to maintain a selected deposition rate with as low as 2% of a monolayer per second. 
     Source  702  is also equipped with an evaporation/sublimation source  722 . For example, resistively heated crucible  722  containing a powder of molecular precursor  724  such as HfCl 4 . The sublimation source is mounted within source  702 . Electrical-current feedthroughs  720  are used to feed power to provide quick heating of crucible  722 . Commercially available crucibles are proven to be able to initiate well controlled thermal evaporation/sublimation of various materials within a time scale of seconds when feedback controlled to sustain a preset reading on a deposition rate sensor such as sensor  710 . For example, the feedback loop schematically illustrated in  FIG. 19  with comparator  728 , control lines  718  and  726  and set-point  730 . In the case of chemical source  700 , sensor  710  preferably does not have a line of sight with the vapors emerging from sublimation source  722 . 
     Source  700  also includes a refill valve  734  and a refill conduit  732 . To replenish the source chemical, for example HfCl 4  powder, the powder is preferably immersed within an inert liquid to generate a slurry. The chemical is loaded into an idle, vented, and preferably cold (i.e., room temperature) source  700  from point  736  by opening valve  734 . Following the delivery of a metered amount of slurry, valve  734  is flushed with sufficient amount of inert liquid to flush the powder from the valve prior to shutting the valve off. The source is then evacuated and the inert liquid is subsequently evaporated using vacuum pump  742  through valves  734  and  740  and line  738 . For this operation, slurry-source manifold and vent manifold that are connected at point  736  are isolated using appropriate shut-off valves (not shown). During inert liquid evaporation, the sublimation source may be slightly heated to promote rapid removal of the liquid. 
     The solid material is preferably introduced as a thoroughly mixed slurry of fine powder solid chemical with an inert, highly volatile liquid such as freon, carbon tetrachloride (CCl 4 ), trichloroethylene, or Galden HT55 from Solvag Solexis, to name a few alternatives. The liquid is used to shield the precursor from ambient exposure during transfer since most precursors react violently with moisture and/or oxygen. A slurry is better suited than a solvent since it is more generic. In addition, solvated precursor can still react with the ambient while wet, and immersed solid particles in a slurry are practically isolated from contact with the ambient. A slurry is also easier to dry-out completely from all trace of liquid. 
     The source  700  illustrated in  FIG. 19  can appropriately control the pressure of a solid as well as liquid chemical with consistency that is well suited for ALD. In another embodiment, source  700 ′ that is illustrated in  FIG. 20  controls both the partial pressure of a solid precursor or a liquid precursor as well as the total pressure of precursor diluted within inert carrier gas. For this purpose, the partial pressure of precursor is maintained and feedback controlled using the combination of sensor  710  and sublimation/evaporation source  722  and an appropriate deposition controller  716  as described above with reference to source  700  in  FIG. 19 . In addition, high-temperature pressure gauge  706  and valve  744  are used to deliver inert gas from inert gas manifold  746  to maintain source volume  702  at a controlled total pressure. Shutoff valve  744  is feedback controlled to maintain set-point total pressure  750 . Alternatively, high temperature proportional valve in combination with shutoff valve  744  may be used. The total pressure is controlled to exceed the vapor pressure from the chemical. Since the additional gas in not condensable at the temperature of the QCM, the QCM is able to control the partial pressure of the chemical independent of the total pressure. 
     Source  700 ′ advantageously allows one to prepare and sustain a well-controlled mixture of reactive chemical vapor seeded into inert carrier gas wherein both the partial pressure of the precursor and the total pressure are independently controlled. In addition, source  700 ′ can be used for in situ preparation of precursor. Accordingly, source  700 ′ is operated in an etch mode wherein the gas supplied from manifold  746  contains an etch gas such as Cl 2 , Cl 2 /O 3 , HF, etc., or a well-optimized mixture of etch gas with inert gas. The sublimation source  722  contains a metallic powder or otherwise a compound suitable for effective generation of volatile etch product (such as RuO 2  suitable to generate Ru(VIII) oxide or ruthenium oxichlorides). The temperature of crucible  722  is controlled to promote efficient reaction of etch gas with the target material  724  to generate the desired molecular precursor. Using this method eliminates the need to handle and contain hazardous chemicals within source  700 ′ and in the slurry manifold. In addition, metallic sources are usually available at substantially higher purity than compound chemicals and at substantially lower cost. Additional advantage comes from the ability to generate metastable precursor molecules which are otherwise difficult to store such as the oxichlorides of Ta, Nb, W, Hf, Zr, Ru, etc., as well as volatile precursors that are unstable and extremely hazardous such as RuO 4 . For example, HfCl 4  may be generated from the combination of Hf powder within crucible  722  and a mixture of Cl 2 /N 2  delivered from  746 . The temperature of  722  is controlled to satisfy a low condensation rate of HfCl 4 , clearly the most volatile etch product, on sensor  710 . In an alternative method the precursor HfOCl 2  is generated from metallic Hf in  722  and a mixture of Cl 2 /O 2 /O 3  coming from manifold  746 . In this case, the volatile HfOCl 2  will be the dominant etch product at sufficiently high ratio of O 3 /Cl 2 . In yet another example, RuO 4  is generated from RuO 2  loaded into  722  and O 2 /O 3  delivered from manifold  746 . In yet another example, volatile carbonyl molecules such as W(CO) 6  are prepared using W powder and CO gas. 
     In alternative embodiment, source  700 ″′ illustrated in  FIG. 21  implements an alternative sublimation source  770  using metallic target  722  shaped as a wire (shown), rod, plate, foil etc. wherein the sublimation of in-situ prepared precursor from etching target  772  is promoted by running direct heating current through target  772  from transformer  778  that is controlled to maintain the desired low condensation of the precursor on sensor  710 . In the configuration shown in  FIG. 21 , a high purity wire  772  is clamped using high purity metallic clamps  774 . 
       FIG. 22  presents another exemplary embodiment wherein source  700 ″′ does not include a pressure gauge to control the total pressure within source  700 ″′ internal volume. Reliable pressure gauges such as the MKS  631 A are available and can withstand harsh chemicals and temperature up to 200° C. These gauges are well suited for sources such as in  700 ′ (FIG.  20 ) and  700 ″ ( FIG. 21 ). However, in some cases sources temperature exceeding 200° C. may be desired. In addition, the combination of high-temperature and corrosive chemicals may accelerate drift and failure of pressure gauges such as the MKS  631 A or the MKS  628 B. Additionally, high temperature pressure gauges are rather expensive. In the alternative approach of  FIG. 22 , the total pressure is indirectly maintained by providing a well pressure controlled reservoir  780  upstream to valve  744 ′. The volume of reservoir  780  is sufficiently large to sustain well regulated pressure with the aid of an off-the shelf pressure controller  782  such as the MKS series  640 A devices. The gas is fed at point  746 ′ and maintained within reservoir  780 . Following the completion of a dose from source  700 ′″, the total pressure is replenished by opening valve  744 ′ for a duration sufficiently long to substantially bring the pressure within  700 ″′ internal volume to the pressure that is controlled within  780 . Preferably, the opening of valve  744 ′ is not timed excessively long to avoid diffusion of non-volatile precursor upstream of valve  744 ′ where the temperature is not sufficiently high to prevent condensation. While the sublimation source shown in  FIG. 22  resembles the source of  FIG. 21 , it is understood that the method for total pressure maintenance taught with reference to  FIG. 22  is suitable for the embodiments and method that were presented in reference to  FIG. 20  and their derivatives such as direct sublimation or in-situ precursor generation, as well. 
     The apparatuses for in-situ generation of precursors using the method that was described in reference to  FIGS. 20 ,  21 , and  22  is suitable to prepare desired compound precursor molecules that are substantially more volatile than the pure element or other undesired compounds. Many different precursors can be prepared with substantial advantages ranging from stability, safety, consistency, purity and cost. Several examples are given below (for simplicity, the equations are not balanced): 
       Hf+Cl 2 →HfCl 4 ↑  (1) 
       RuO 2 +O 2 +O 3 +Cl 2 →RuO x Cl 4-x ↑  (2) 
       W+Cl 2 →WCl 6 ↑  (3) 
       Ru+CO →Ru 3 (CO) 12 ↑  (4) 
       Mo+Cl 2 →MoCl 5 ↑  (5) 
     Systems, apparatuses, and methods designed and operated in accordance with the invention are particularly useful in ALD technology. Synchronous modulation of flow and draw, SMFD, is also useful, however, in a wide variety of circumstances and applications. It is evident that those skilled in the art may now make numerous uses and modifications of the specific embodiments described, without departing from the inventive concepts. It is also evident that the steps recited may, in some instances, be performed in a different order; or equivalent structures and processes may be substituted for the structures and processes described. Since certain changes may be made in the above systems and methods without departing from the scope of the invention, it is intended that all subject matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense.