Continuous substrate processing system
09748125 · 2017-08-29
Assignee
Inventors
- Banqiu Wu (Sunnyvale, CA, US)
- Nag B. Patibandla (Pleasanton, CA, US)
- Toshiaki Fujita (Sakura, JP)
- Ralf Hofmann (Soquel, CA, US)
- Pravin K. NARWANKAR (Sunnyvale, CA, US)
- Jeonghoon Oh (San Jose, CA, US)
- Srinivas Satya (Sunnyvale, CA, US)
- Li-Qun Xia (Cupertino, CA, US)
Cpc classification
C23C16/45546
CHEMISTRY; METALLURGY
H01L21/67346
ELECTRICITY
C23C16/54
CHEMISTRY; METALLURGY
International classification
C23C16/00
CHEMISTRY; METALLURGY
C23C16/458
CHEMISTRY; METALLURGY
C23C16/54
CHEMISTRY; METALLURGY
H01L21/00
ELECTRICITY
C23C16/455
CHEMISTRY; METALLURGY
Abstract
A processing chamber having a plurality of movable substrate carriers stacked therein for continuously processing a plurality of substrates is provided. The movable substrate carrier is capable of being transported from outside of the processing chamber, e.g., being transferred from a load luck chamber, into the processing chamber and out of the processing chamber, e.g., being transferred into another load luck chamber. Process gases delivered into the processing chamber are spatially separated into a plurality of processing slots, and/or temporally controlled. The processing chamber can be part of a multi-chamber substrate processing system.
Claims
1. A processing chamber for processing a plurality of substrates, the processing chamber comprising: a chamber body having a top end and a bottom end with a first opening near the bottom end and a second opening near the top end; a plurality of substrate carriers between the first opening and the second opening, each of the plurality of substrate carriers having a plurality of spacer pins positioned on an outer circumference of each of the plurality of substrate carriers to provide a vertical stack of substrate carriers with a space between each substrate carrier; a gas delivery system; an exhaust system; a plurality of processing slots formed between the bottom end and the top end of the chamber body, wherein each of the plurality of processing slots is in fluid communication with a fluid delivery channel of the gas delivery system and a fluid outlet of the exhaust system to deliver a horizontal flow of gas across each of the plurality of processing slots; a substrate latch comprising at least three movable contacting latches positioned near the bottom end of the chamber body and above the first opening, the at least three movable contacting latches configured to support the vertical stack of substrate carriers; and a substrate support assembly to secure and support an additional substrate carrier and load the additional substrate carrier into the processing chamber from a bottom of the vertical stack of substrate carriers, wherein loading an additional substrate carrier comprises elevating the vertical stack of substrate carriers with the substrate support assembly to a position above the first opening, moving the contacting latches to a support position, the movable contacting latches in the support position circumferentially supporting the vertical stack of substrate carriers, lowering the substrate support assembly to a position to support the additional substrate carrier, adding the additional substrate carrier onto the substrate support assembly, moving the substrate support assembly upward with the additional substrate carrier to contact the bottom of the vertical stack of substrate carriers, moving the movable contacting latches from the support position to a non-supporting position so that the substrate support assembly with the additional substrate carrier supports the vertical stack of substrate carriers.
2. The processing chamber of claim 1, wherein each of the plurality of processing slots processes one single substrate at a time.
3. The processing chamber of claim 1, wherein each of the plurality of substrates is transferred inside the chamber body one at a time.
4. The processing chamber of claim 1, wherein a substrate can be transferred into the chamber body via the first opening, processed from one processing slot, continuously moved to another processing slot within the chamber body, and transferred out of the chamber body via the second opening.
5. The processing chamber of claim 1, wherein a volume for each processing slot has a height corresponding to the distance between two movable substrate carriers.
6. The substrate processing system of claim 1, further comprising a transfer robot to transfer a substrate supported on the additional substrate carrier into the processing chamber via the first opening for processing the substrate through the plurality of processing slots.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
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DETAILED DESCRIPTION
(13) Embodiments of the present invention generally provide a substrate processing chamber for maximizing throughput and minimizing system footprint. The processing chamber is designed to be used in a continuous multi-substrate processing method and tailored to meet tighter process window requirements of existing and new ALD and CVD substrate deposition applications. In general, the chamber volume of the processing chamber is large enough to concurrently process multiple substrates. Each of the multiple substrates is processed, continuously and one after another, inside the processing chamber, and high-quality film can be deposited and consistently obtained on each and every one of the multiple substrates.
(14) In some embodiments, the throughput is improved by using the substrate processing chamber as described herein to load, unload, and process multiple substrates continuously and save time in loading, unloading and processing each individual substrate. For example, the throughput (the number of substrates processed per hour) of such a processing chamber can be about 20 times (20×) or higher of the throughput of a single-wafer substrate processing system or about 6 times (6×) of the throughput of a conventional multi-substrate processing system. Conventionally, a long waiting time is common when a processing chamber is idle and waiting for substrate loading and unloading, prior to and after substrate processing. Here, a continuous process is designed for a substrate processing chamber where the chamber idle time can be avoided and saved because, as a plurality of substrates are processed inside the substrate processing chamber, there are other substrates being continuously loaded in and unloaded out of the substrate processing chamber.
(15) In addition, throughput of the substrate processing chamber is also improved in such a continuous process by not sacrificing the number of substrates processed per hour during pre-treatment and post-treatment of the substrates. For example, the substrate processing chamber is adapted to pre-heat each substrate prior to substrate processing and the time in ramping up the temperature of each substrate does not affect its throughput.
(16) In some embodiments, the interior portion of the processing chamber is configured with multiple processing slots for processing a plurality of substrates and each processing slot is minimized to reduce footprint and accommodate, one after another, each substrate. The effective processing volume in each processing slot is kept to a minimum volume. Together, high throughput and small effective processing volume in the processing slots function to improve film deposition rate for each substrate.
(17) In addition, once loaded inside the processing chamber, each substrate is designed to be processed through each and every one of the multiple processing slots. Accordingly, substrate processing uniformity and deposited film quality are improved because each substrate is moved and passed through each of the processing slots and encountered the same processing environment and conditions when traveled continuously through the interior portion of the processing chamber. It is observed that the properties of a deposited film on a substrate processed by the processing chamber are comparable or better than those by a single-wafer processing system.
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(19) In one or more embodiment, the processing chamber 200 is capable of loading at least one single substrate 210 from one end of the chamber body 202 (e.g., via the first opening 220 positioned near the bottom end of the chamber body 202) and unloading another substrate from another end (e.g., via the second opening 230 positioned near the top end of the chamber body 202). In some embodiments, each of the plurality of the substrates 210 is transferred inside the chamber body 202, one substrate at a time. In one or more embodiment, the processing chamber 200 is adapted to concurrently transfer/load one substrate inside the chamber body 202 and transfer/unload another substrate out of the chamber body 202.
(20) In some embodiments, at least one substrate 210 is disposed on the surface of a substrate carrier 240, and a single substrate carrier 240 carrying a substrate 210 can be movably transferred in and out of the processing chamber 200. The substrate carrier 240 carrying the substrate 210 thereon may be transferred into the processing chamber 200 in a substrate-in direction 222 (e.g., in a horizontal direction and via a slit valve near the first opening 220).
(21) In addition, a plurality of spacer pins 242 are positioned on the outer circumference of each substrate carrier 240 to better receive each substrate 210. For example, three or more spacer pins 242 may be disposed on the substrate carrier 240 to surround the outer edges of the substrate 210 and prevent slippage of the substrate 210 during substrate transfer. In some embodiments, multiple substrate carriers 240 are vertically spaced apart via three or more spacer pins 242 located on the outer circumference of each substrate carrier 240. In some embodiments, the spacer pins 242 in-between the stacked substrate carriers 240 are positioned to separate the substrates 210 disposed on the stack of multiple substrate carriers 240 within the processing chamber 200. In addition, the spacer pins 242 provide a space between the substrates 210 and the substrate carrier 240 stacked above the substrates 210. In some embodiments, the surface of each substrate 210 disposed on the surface of the substrate carrier 240 and the height of the spacer pins 242 between two stacked substrate carriers 240 are provided to define a space for each processing slot 280 within the internal region 208 of the processing chamber 200.
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(23) In one or more embodiment, the substrate support assembly 244 includes a shaft 243 and a support mechanism 241 such that the substrate support assembly is able to move upwardly and downwardly (e.g., in a direction 272) and is capable of horizontal rotary movement (e.g., in a clockwise or counter-clockwise direction 274). In one or more embodiment, the substrate support assembly 244 may include a heating system 206 embedded therein for pre-treatment (e.g., pre-heating, among others) of a newly loaded substrate. The heating system 206 may include embedded resistive elements. Other heating systems adapted for radiant or convective heating of the substrates 210 and/or substrate carriers 240 disposed above the substrate support assembly 244 may also be used. Optionally, the surface of the substrate support assembly 244 may also include three or more spacer pins 242 located on its outer circumference for receiving the substrate carrier 240 disposed thereon.
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(25) Next, the newly loaded substrate carrier 240 having the substrate 210 disposed thereon is received by a substrate-latch mechanism 245 positioned near the bottom end of the chamber body 202. The substrate-latch mechanism 245 may include three or more movable contacting latches positioned around the edges of the substrate carrier 240 for supporting the newly loaded substrate carrier 240 near the bottom end of the chamber body 202. In addition, once a plurality of substrate carriers 240 are loaded above the substrate support assembly 244 and stacked upwardly, one by one, inside the processing chamber 200, the substrate-latch mechanism 245 is capable of supporting a plurality of the substrate carriers 240 having a plurality of the substrates 210 thereon.
(26) As a result, each of the plurality of the substrates 210 is disposed on and carried by one substrate carrier 240, and the plurality of the substrate carriers 240 are stacked vertically one on top of another within the chamber body 202 of the processing chamber 200. Accordingly, the plurality of the substrates 210 are disposed on the surface of the substrate carriers 240 to be concurrently secured and supported by the substrate-latch mechanism 245 and continuously and upwardly processed within the internal region 208 with continuous substrate loading onto the substrate support assembly 244. Thus, two or more substrate carriers 240 are vertically stacked, one on top of another one, within the processing chamber 200.
(27) As shown in
(28) Each processing slot 280 is connected to a gas distribution system and a gas exhaust system. As shown in detail in
(29) In another embodiment, as shown in
(30) The substrate support assembly 244 and/or the substrate-latch mechanism 245 are capable of moving upwardly and downwardly (e.g., at least for the vertical distance, Dv, for loading and moving upwardly a newly loaded substrate carrier 240 to the bottom of the stack of the substrate carriers 240) such that the plurality of the substrates 210 previously disposed inside the processing chamber 200 are moved together upwardly for the distance, Dv, from one processing slot 280 to the next processing slot 280.
(31) Optionally, the substrate support assembly 244 may horizontally rotate in the direction 274 (e.g., clockwise or counter-clockwise for a degree of more than zero and less than 360°) and move the stack of the plurality of the substrate carriers 240, together with the substrates 210 disposed thereon, in order to obtain uniform horizontal gas distribution through each of the processing slots 280 when multiple substrates 210 are concurrently and continuously processed inside the processing chamber 200.
(32) In one or more embodiment, once a new substrate is loaded near the bottom of the stack of the plurality of the substrate carriers 240, the substrates already positioned within the processing chamber 200 and the newly-loaded substrate can all be supported by the substrate-latch mechanism 245. Then, the substrate support assembly 244 can move downwardly in the direction 272 to receive another substrate 210 carried by another substrate carrier 240.
(33) Referring back to
(34) In one aspect, the height of the volume for each processing slot 280 (e.g., the vertical distance, Dv, between two processing slots 280) corresponds to the distance between two movable substrate carriers 240 loaded inside the processing chamber 200 and supported by the substrate-latch mechanism 245. For example, the height of the plurality of the spacer pins 242 may correspond to the height of each processing slot 280 within the processing chamber 200 (e.g., the height of the spacer pins 242 as shown in
(35) Accordingly, the volume of each processing slot 280 can be kept minimal and such small volume enables each processing step performed in each processing slot 280 to be more efficient. In another aspect, by vertically stacking up multiple movable substrate carriers 240 within the processing chamber 200, multiple processing slots 280 are formed vertically between the first opening 220 and the second opening 230 of the batch substrate processing chamber 200 to reduce chamber footprint and keep the batch substrate processing chamber 200 vey compact for processing multiple substrates 210 in the multiple processing slots 280. In one aspect, each processing slot 280 within the processing chamber 200 is configured for receiving and processing one single substrate at a time.
(36) When each substrate carrier 240 carrying the substrate 210 thereon is newly transferred inside the internal region 208 of the chamber body 202, it is continuously stacked to the previous substrate carrier 240 and moved (e.g., vertically and upwardly) through each of the multiple processing slots 280, one by one, before being transferred out of the processing chamber 200. The substrate 210 is thus being processed through each of the multiple processing slots 280.
(37) Each processing slot 280 is in fluid communication with a gas delivery system and/or an exhaust system (e.g., fluid delivery channels 252, fluid outlets 262, and an exhaust system 260 as discussed in
(38) Accordingly, each substrate 210, once positioned on the substrate carrier 240 and transferred inside the processing chamber 200, is continuously moved through each of the multiple processing slots 280 within the internal region 208 of the processing chamber 200, where a semiconductor process (e.g., ALD and other epitaxial process) is continuously performed in multiple times (e.g., cyclically as described in further detail in
(39) In general, the processing chamber 200 is used for performing at least one semiconductor process (e.g., CVD, PVD, ALD, annealing, among others) on a plurality of substrates 210. In one or more embodiment, the processing chamber 200 is designed to meet the needs of the process regime of an atomic layer deposition (ALD) process under controlled temperature and pressure. For example, each processing step of a cyclical ALD process may be performed spatially in each of the processing slots 280 of the processing chamber 200. Alternatively, each processing step of a cyclical ALD process may be controlled temporally by delivering of gas flows in pulses (e.g., pulsing of gas flows of various precursor gases, inert carrier gases, and flows of purge gases for short, repeated time periods). As an example, the processing chamber 200 may be used to perform an ALD cyclical process on a group of substrates, where each substrate is processed sequentially through different steps or stages of a typical ALD cyclical process. As another example, the processing chamber 200 may be configured to perform the same process simultaneously on a group of substrates. In one or more embodiment, each of the multiple processing slots 280 can be configured for performing a single step (or stage) of an ALD cyclical process. In another embodiment, the multiple processing slots 280 can be selectively configured for performing the same or different steps of an ALD cyclical process within a defined time period.
(40) Accordingly, the processing chamber 200 is configured with multiple processing slots (e.g., the processing slot 280) with each processing slot 280 is spaced between two stacked substrate carriers 240. The processing slot 280 can thus be compact and minimized. The amounts of expensive precursor compounds to be delivered into the processing slot 280 can be reduced. In addition, process conditions are easier to maintain in a small space (with smaller volume).
(41) In one or more embodiment, the processing chamber 200 is a stand-alone substrate processing system. In another aspect, the processing chamber 200 can be coupled to a multi-chamber substrate processing system and connected to additional chambers (e.g., additional process chambers, transfer chamber, and/or load luck chambers 120, 180). In one or more embodiment, the processing chamber 200 is coupled to two load luck chambers 120, 180 to be capable of continuously and/or simultaneously loading the substrate 210 from one end of the chamber body 202 and unloading a different substrate from another end of the chamber body 202. For example, the load luck chamber 120 can be used to load the substrate carrier 240 carrying the substrate 210 to be processed from the lower end of the chamber body 202, whereas the load luck chamber 180 can be used to unload another substrate from the upper end of the chamber body 202. In one aspect, the loading and unloading of the substrate-carrying substrate carriers 240 are performed at the same time (concurrently or simultaneously) to improve system throughput and maintain consistent substrate processing conditions inside the processing chamber 200.
(42) The processing chamber 200 may be coupled to a multi-chamber substrate processing system, which may include one or more pre-processing treatment chambers, post-processing treatment chambers, heating chambers, transfer chambers with one or more transfer robots, wafer storage cassettes, a factory interface, and/or an utility tower, among others, A system controller is connected to the processing chamber 200 and/or the multi-chamber substrate processing system for controlling the process performed inside the processing chamber 200. In some embodiments; a transfer robot is required to load and unload the substrate carrier 240 into the processing chamber 200.
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(44) A fluid delivery system is connected to the processing chamber 200 for introducing process gases (e.g., reactive precursor gases, fluids, carrier gases, nonreactive inert gases, and other reactive or non-reactive gases and fluids) into the processing slot 280. For example, the fluid delivery system may deliver one or more flows of process gases into each processing slot 280.
(45) As shown in
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(47) In some embodiments, two or more sets of fluid delivery channels 252, which are connected to a fluid source, may be disposed on the vertical side of the bell jar body 203 to deliver process gases into the two or more processing slots 280A, 280B. Two or more valves 254 may be used to control the flows of process gases into the processing slots 280A, 280B.
(48) In addition, two or more sets of fluid outlets 262, which are connected to an exhaust system, may be disposed on the vertical side of the bell jar body 203 (e.g., on the opposite side of the fluid delivery channels 252 to deliver process gases out of the two or more processing slots 280A, 280B. Two or more valves 264 may be used to control gas flows out of the processing slots 280A, 280B.
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(50) In one example, the substrate lifting assembly may include three substrate lift mechanisms 245A, 245B, 245C, to circumferentially support a first set of the substrate carriers 240, and three substrate lift mechanisms 247A, 247B, 247C, to circumferentially support another set of substrate carriers 240 inside the processing chamber 200. For example, when a substrate carrier is newly transferred inside the processing chamber 200 and moved up be the substrate support assembly 244 to be latched by the substrate-latch mechanism 245, the three substrate lift mechanisms 245A, 245B, 245C may move horizontally sideways for a distance, DH, and vertically downward for a distance, Dv, to catch the newly latched substrate carrier, while the three substrate lift mechanisms 247A, 247B, 247C are supporting the rest of the substrate support already stacked the processing chamber 200. Then, the substrate lift mechanisms 245A, 245B, 245C, 247 A, 247B, 247C supporting all of the stacked substrate carriers 240 may move vertically upward for a distance, Dv, such that the substrate 210 on the stacked substrate carriers 240 can be moved vertically upward a distance, Dv, to be processed in the next processing slot 280. In this configuration, each substrate 210 is able to be processed sequentially from processing slot 280A located near the first opening 220 and upwardly through processing slots 280B, 280C, 280D, 280E, etc.
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(52) Exemplary suitable materials that can be deposited on the surface of the substrate 210 disposed inside the processing chamber 200 include, but are not limited to, various nitrides, oxides, carbides, among others. For example, for depositing a tantalum nitride (TaN) material layer over a surface of the substrate 210, two precursor compounds may be used. The first precursor may be a tantalum containing compound, such as a tantalum based organo-metallic precursor or a derivative thereof, e.g., pentadimethylamino-tantalum (PDMAT; Ta(NMe.sub.2)s), pentaethylmethylamino-tantalum (PEMAT; Ta[N(C.sub.2H.sub.5CH.sub.3).sub.2].sub.5), pentadiethylaminotantalum (PDEAT; Ta(NEt.sub.2).sub.5), TBTDET (Ta(NEt.sub.2).sub.3NC.sub.4H.sub.9 or C.sub.16H.sub.39N.sub.4Ta) and tantalum halides, and any and all of derivatives of the above listed compounds. The tantalum containing compound may be provided as a gas or may be provided with the aid of a carrier gas. Examples of carrier gases which may be used include, but are not limited to, helium (He), argon (Ar), nitrogen (N.sub.2), and hydrogen (H.sub.2).
(53) After the delivery of the first precursor gas (precursor gas A) into the processing slot 280 of the processing chamber 200, a monolayer of the tantalum containing compound is chemisorbed onto the surface of the substrate 210 and excess tantalum containing compound is removed from the process chamber by introducing a pulse of a purge gas thereto. Examples of purge gases which may be used include, but are not limited to, helium (He), argon (Ar), nitrogen (N.sub.2), hydrogen (H.sub.2). and other gases.
(54) After the process chamber has been purged, a second precursor gas (precursor gas B) may be delivered into the processing slots 280 of the processing chamber 200. The second precursor may be a nitrogen containing compound with nitrogen atoms and one or more reactive atoms/species. For example, the nitrogen containing compound may be ammonia gas (NH.sub.3) and other nitrogen containing compounds, including, but not limited to, N.sub.xH.sub.y with x and y being integers (e.g., hydrazine (N.sub.2H.sub.4)), dimethyl hydrazine ((CH.sub.3).sub.2N.sub.2H.sub.2), t-butylhydrazine (C.sub.4H.sub.9N.sub.2H.sub.3) phenylhydrazine (C.sub.6H.sub.5N.sub.2H.sub.3), other hydrazine derivatives, a nitrogen plasma source (e.g., N.sub.2, N.sub.2/H.sub.2, NH.sub.3, or a N.sub.2H.sub.4 plasma), 2,2′-azoisobutane ((CH.sub.3).sub.6C.sub.2N.sub.2), ethylazide (C.sub.2H.sub.5N.sub.3), and other suitable gases. The nitrogen containing compound may be introduced into the processing slot 280 as a pulse, and may be provided alone. Alternatively, a carrier gas may be used to deliver the nitrogen containing compound if necessary.
(55) After the delivery of the second precursor gas (precursor gas A) into the processing slot 280 of the processing chamber 200, a monolayer of the nitrogen containing compound may then be chemisorbed on the monolayer of the tantalum containing compound. The composition and structure of precursors on a surface during atomic-layer deposition (ALD) is not precisely known. Not wishing to be bound by theory, it is believed that the chemisorbed monolayer of the nitrogen containing compound reacts with the monolayer of the tantalum containing compound to form a tantalum nitride layer. Reactive species from the two precursor compounds may form by-products that are transported from the substrate surface (e.g., via the fluid outlets 262 and the exhaust system 260). It is believed that the reaction of the nitrogen containing compound with the tantalum containing compound is self-limiting and, in each pulse of delivering a precursor compound into the processing slot 280, only one monolayer of the precursor compound is chemisorbed onto the surface of the substrate 210. Each cycle of the sequential delivery of the two or more alternating precursors over the surface of the substrate is repeated (e.g., 20-30 cycles) until a desired thickness of the material layer (e.g., a tantalum nitride film) is formed.
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(61) The loading stage 810 is configured to load one substrate 210 (e.g., being carried on a substrate carrier 240) into the processing chamber 200. The loading stage 810 includes a step 812 for moving down a piston to receive the substrate 210 disposed on the substrate carrier 240, a step 814 for moving the substrate 210 inside the processing chamber 200, a step 816 for latching the substrate onto the substrate-latch mechanism 245, and a step 818 for moving the substrate up (e.g., by the use of one or more substrate lift mechanisms 245A, 245B, 245C, 247A, 247B, 247C) within the internal region of the processing chamber 200.
(62) The processing stage 820 is configured to process one substrate 210 in each of the processing slots 280. For example, by vertically moving the substrate 210 sequentially through the processing slot 280A, 280B, 280C, 280D, and 280E. The processing stage 820 includes a step 822 for moving a substrate up (e.g., by the use of one or more substrate lift mechanisms 245A, 245B, 245C, 247A, 247B, 247C).
(63) Each substrate 210 is then continuously and sequentially exposed to a flow of purge gas at step 824 (in pulses or a constant gas flow), a flow of precursor A at step 825 (with or without the aid of a purge gas or carrier gas), and a flow of precursor 8 at step 826 (with or without the aid of a purge gas or carrier gas). Next, at step 828, the substrate 210 is continuously being moved vertically upward through the processing slots 280 and at the same time exposed to cycles of gas flows of precursor A, purge gas, precursor B, and purge gas.
(64) Next, the unloading stage 830 is configured to unload one substrate 210 (e.g., being carried on the substrate carrier 240) outside the processing chamber 200. The unloading stage 830 includes a step 832 for latching the substrate disposed on the substrate carrier 240 by a transfer mechanism (e.g., a transfer robot) and a step 834 for moving the substrate 210 outside the processing chamber 200 by the transfer mechanism.
(65) In one or more embodiment, the processing chamber 200 is configured to process a plurality of the substrate 210 simultaneously, for example, by concurrently performing each loading stage 810, processing stage 820, and unloading stage 830 on a number of substrates inside the processing chamber 200. In additional, loading of one substrate and unloading of another substrate can be performed simultaneously to save time and prevent interference of the ALD processing of additional substrates inside the processing chamber 200.
(66) While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.