Patent Application
20090304924
The present invention is in the area of apparatus and methods for chemical vapor phase processing of multi-layer thin films of various materials at one atomic layer precision. More, particularly, this invention relates to processing of multi-layer thin films at one atomic layer precision on flexible substrates at high-speed for manufacturing of semiconductor devices, large area thin-film photovoltaic solar cells, flexible displays and catalytic electrodes for fuel cells, among other applications.
Thin film processing forms a critical part of fabrication of a variety of advanced devices such as microelectronic devices, optoelectronic and photonic devices, thin film photovoltaic solar cells and optical coatings and so on. In all these applications, invariably large-area processing uniformity along with high speed of processing is important to achieve economics of scale. A variety of techniques of thin film deposition such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) are currently being practiced in the industry to deposit thin films of metals, semiconductors and insulators. These and associated techniques of thin film deposition are flux dependent and thus can offer much desired thin film uniformity on larger area substrates with significant challenges in the apparatus design and its operation and at higher cost. Although these techniques can deposit thin films at a high rate, ranging from several tens of nm/min to a few hundred nm/min., a glaring shortcoming is an inability to deposit high quality and conformal thin films in narrow, sub-micron geometrical features and film higher film thickness uniformity that is exceedingly difficult to achieve with increasing substrate area.
These inadequacies in the prevalent thin film deposition techniques are largely removed by a recent entrant to thin film deposition processes which is know in the art as Atomic Layer Deposition, or simply ALD, which was invented in mid 1970s but was not applied to microelectronic device fabrication until recently. ALD is a variant of CVD. An ALD process, based on a well-known principle of chemisorption, forms a strongly adherent monolayer of reactive gas molecules, and is thus self limiting and also independent of the area of the substrate. Moreover, ALD thin films are typically conformal, even in deep, sub-micron geometries of a substrate surface morphology. These are extremely powerful and much sought-after attributes for a variety of advance device processes. In a typical ALD process, excess physiosorbed gas molecules of the reactive gases, and also reaction by-products, are swept away by inert gas pulses from the vicinity of the substrate. All the gases are sequentially injected and spread over the entire substrate surface to form a monolayer of the thin film to be deposited. The substrate is appropriately heated or supplied in another manner with necessary activation energy to affect the overall reaction of chemisorption and chemical reaction to form the product thin film. An ALD process thus typically consists of four pulses including two inert gas pulses. The overall process sequence of four gas pulses is repeated to build the desired film thickness in a cyclic manner with a monolayer precision. A typical ALD process is schematically represented by a generic chemical reaction as shown in equation 1 below:
In an ALD process, the second pulse of inert gas P in reaction described above in equation (1) that is responsible for creating a chemisorbed monolayer of Ax2 type specie on the surface of the substrate which then further reacts with reactive gaseous species By2 to form a monolayer AB. The fourth pulse of the inert gas P removes the reaction by-products xy and also any excess of By2. The inert gas pulses in the reaction described in equation (1) can be replaced by simple evacuation of the process chamber in order to remove excess of reactant Ax2 from the substrate surface, however, impingement of an inert gas improves the efficiency of dislodging the excess reactive gas Ax2 (and also By2 and the by-product of the overall chemical reaction—xy) from the vicinity of the substrate by imparting momentum. Moreover, an inert gas does not actively participate in the overall chemical reaction.
Yet another method of practicing an ALD process is to maintain a constant flow of inert gas in the chamber and intersperse reactive gas pulses with a sufficient time span between them. At the same time, removal of the second pulse or the fourth pulse or both of the inert gas (or even mere evacuation steps employed in lieu of inert gas pulses), will transform the overall reaction mechanism from an ALD type to high-rate CVD type. The overall process comprising four pulses should have been appropriately termed as a monolayer deposition process. However, atomic layer deposition has been prevalent since its inception and accepted thereafter. Fundamentals of the ALD process and also basic reactor hardware are described in U.S. Pat. No. 4,058,430 by Suntola et al., which is included in the present specification by reference.
In practice a typical ALD process is quite slow as compared to a conventional CVD process because the ALD process critically depends on the time taken to complete one ALD process cycle. The cycle time in ALD in turn depends upon the gas residence time (and also on non-turbulent gas flow) within the ALD reactor, in addition to the speed of operation of the gas pulsing mechanism and hardware, e.g., fast switching valves, their efficiency, reliability and useful lifetime are important factors. The gas residence times in an ALD reactor, which depend upon reactor volume, operating pressure and the gas flow rate, for large substrates measuring as much as 1 square meter, may reach up to several seconds, with resulting deposition rate of barely 1 nm/min. As a result, practical application of ALD to large area substrates is restricted to very thin films—such as a few tens of nanometers or below. This situation may be partly remedied by employing batch processing. However, batch processors are undesirable due to a variety of factors as substrate backside deposition, proportionately larger volume, and substrate load-unload time. Thus, the industry trend has been in favor of smaller-volume, single-substrate, or a mini-batch (four to five substrates per batch) ALD reactors. Therefore, for an efficient ALD reactor operation, a judicious choice has to be made by comparing the pros and cons of all these factors and their impact on the ALD cycle time to arrive at an optimized solution for a required application.
Multiple-wafer batch processors that can block backside deposition on the substrates have been prevalent in the area of epitaxial thin films of silicon and compound semiconductor thin films such as GaAs and others. One of the most successful reactor configurations for this purpose has been a barrel type reactor in which a number of substrates are mounted on the faces of a trapezoid shaped solid susceptor. The susceptor may be heated by means such as external lamps or by an inductive heating arrangement. The reactive gases are typically introduced from the top and flow over substrates before exiting from the bottom of the barrel, while the substrates mounted on the solid susceptor may be rotated around the vertical axis of the susceptor within the barrel. The multi-wafer barrel CVD reactor configuration is a mere extension of the most basic horizontal CVD reactor configuration in which a horizontal CVD reactor is turned through 900 and multiplied around a central axis. Both these reactor configurations are described by Jensen and Kern in Thin Film Processes (II), p. 296-299; J. L. Vossen and W. Kern (eds.), Academic Press, New York, 1991, which is also included herein by reference. The barrel CVD reactor configuration, although useful on small-area substrates, however, is considered inefficient because of the internal gas flow mechanism, which is substantially parallel (longitudinal) to the substrate surface. This flow configuration leads to longer path lengths and thus longer cycle time. It is thus more suitable for CVD type processes. U.S. Pat. No. 5,458,725 describes an arrangement of multiple tubes each with apertures placed parallel to a stationary polygonal susceptor, such that the gas from the tubes is directed in a direction substantially parallel to the stationary susceptor to reduce the particles settling on the substrates attached to the susceptor. U.S. Pat. No. 5,716,484 describes a similar arrangement of multiple vertical tubes, each with a set of apertures on three sides of each tube arranged around a stationary polygonal susceptor. The flow from the tubes impinges on to the stationary susceptor as well as sweeps the inner surface of the barrel to create a swirling flow of gas within the barrel to reduce the particles.
A variety of ALD reactor designs have been introduced to accelerate the overall ALD process by employing multiple and simultaneous processing of substrates in a mini-batch configuration. Kim et al. described an ALD apparatus for simultaneous processing of multiple substrates in U.S. Pat. No. 6,306,216. Recently, multi-wafer continuous-flow ALD reactor configurations with multiple linear injectors for rapid gas distribution on a plurality of wafers rotating around a central axis and being subjected to the gas flow from multiple injectors was described by in U.S. Pat. No. 6,821,563 and U.S. Pat. No. 6,576,062, and also in U.S. Pat. No. 6,634,314. In all these mini-batch ALD reactor configurations multiple substrates are placed on a rotating platform in a horizontal plane that are scanned under linear injectors during rotation. A continuous flow, multi-substrate ALD reactor configuration was described by Bedair et al., for GaAs atomic layer epitaxy process operating at 2.0 micron/h. (˜30 nm/min.) deposition rate who published their results in the Applied Physics Letter, volume 62, No. 19, 10th May 1993. In this ALD reactor configuration, multiple substrates are placed on a susceptor rotating in a horizontal plane and mounted co-axially within a circular chamber which is sub-divided into six equal compartments by quartz partitions. The chamber is supplied with two reactant inlets and an inert gas inlet. The multiple square-shaped substrates mounted on a rotating susceptor are alternatively exposed to the reactants and the inert gas to complete the ALD cycle. Also a multi-wafer ALD reactor configuration with linear injectors mounted above a rotating platen and an atomic layer epitaxial layer process for GaAs thin layers was described by Liu et al., in SPIE volume No. 1676, p. 20 (1992). Use of a laminar flow block comprising multiple linear injectors placed within the outer exhaust port for continuous flow thin film CVD processes in a horizontal conveyer belt configuration is described in the U.S. Pat. No. 5,683,516 and U.S. Pat. No. 6,521,048.
Continuous-flow ALD reactor configurations offer several advantages, such as potentially higher throughput and elimination of complex arrangements of sequencing of gases with fast switching valves. However, the gains that may be realized by multi-wafer ALD reactor configurations can be limited mainly because the reactor volume increases proportionately with the total area of the substrates, thus slowing the overall ALD cycle and the resultant deposition rate. Also, time required to load and unload substrates, which adversely affects the effective throughput, needs to be taken into account. In addition, for batch ALD equipment, the foot-print increases with the number of substrates accommodated, which is yet another factor that requires careful consideration. Furthermore, the substrates such reactors can accommodate are often only planar.
The inherent strengths of an ALD process to offer thin films with low defect density and large area uniformity have been recently applied for fabrication of layers in thin film photovoltaic solar cells. Guillemole et al., reported efficiency of 13.5% in copper indium diselenide (CIGS) solar cells in Japanese Journal of Applied Physics, vol. 40, pp. 6065-6068 (2001); with indium sulfide (In2Se3) buffer layers deposited by ALD. Naghavi et al., reported 16.4% efficiency in copper indium diselenide solar cells comprising a 30 nm buffer layer of indium sulfide (In2Se3) deposited by ALD; in Progress in Photovoltaics, Research & Development, vol. 11, pp. 437-443 (2003). Ohtake et al., reported deposition of 100 nm thick zinc selenide (ZnSe) buffer layer for copper indium diselenide solar cells by ALD in Japanese Journal of Applied Physics, vol. 34, pp. 5949-5955 (1995) with an operating efficiency of 11%. Very recently, however, application of ALD is being increasingly directed towards fabrication of the main absorber layer with the goal to significantly increase the solar cell efficiency. Johansson et al., described an ALD process to deposit copper sulfide layers employing Cu(thd)2 [thd: 2,2,6,6,-tetramethyl-3,5,heptanedione) and H2S as precursors in Journal of Materials Chemistry, vol. 12, pp. 1022-1026 (2002). The rate of deposition at was approx. 0.03 nm/cycle in the temperature range of 125-160 ° C. Recently, Nanu et al., described results of an ALD process to deposit copper indium sulfide (CuInS2) thin films in Chemical Vapor Deposition, vol. 10, No. 1, pp. 45-49 (2004). The precursors employed for the ALD process were cuprous chloride (CuCl), indium trichloride (InCl3) and H2S and the substrates were glass, tin-oxide coated glass and nanoporous TiO2 coated glass with ALD process temperature in the range of 350-500° C. The rate of film deposition, however, at greater than 8 s/cycle, was rather slow for practical use to deposit about a micron thick absorber layer. Very recently, Roscheisen et al., in US Patent Application No. 2005/0186338 described an ALD apparatus for surface treatment on a flexible substrate that is wound around a hexagonal susceptor in a coil form. In US Patent Application No. 2005/0186342, Sager et al., described an apparatus and ALD process to deposit copper indium gallium selenide (CIGS) absorber layer on a long, flexible substrate coiled around a fixed hexagonal shaped susceptor such that the adjacent turns of the coil do not touch one another. All the gases, reactive and inert, are injected into the ALD chamber through a common inlet at the bottom. The goal of such an ALD system configuration is to massively scale the substrate surface area available to the reactants in an ALD cycle to achieve higher throughput as opposed to taking any measure accelerating the ALD cycle speed on to a smaller substrate. Such an ALD system, however, may have to contend with longer substrate load-unload times, inflexibility with respect to gas injection and substantially longer pulse width leading to longer cycle time—in the range of several minutes. For a solar absorber layer about a micron thick, such a processing system may not entirely suitable.
It is thus clear that an atomic layer chemical vapor processing apparatus that can process large area, flexible substrates at significantly higher cycle speed in an ALD and/or CVD mode and also methods of using the same to process a variety of thin films with high degree of uniformity, precision and control on film thickness and composition during the entire process is highly desired. Unique apparatus and methods to accomplish these ends are taught in enabling detail in this specification.
In view of the description of the related art, the present invention describes various configurations of high-speed atomic layer processing apparatus on large area substrates and also methods of operation of such configurations to deposit multi-layer thin films on flexible substrates. The apparatus in the invention in various embodiments is capable of operating at high-speed and within minimum possible foot-print or physical space to uniformly process a substrate or multiple substrates. In the context of this invention, the term atomic layer chemical vapor processing (ALCVP) generally encompasses three processes, namely atomic layer deposition (ALD), atomic layer etching (ALET)—which can be either isotropic or anisotropic, and atomic layer surface modification (ALSM).
Accordingly, the present invention in some embodiments provides an atomic layer chemical vapor processing (ALCVP) reactor that includes a substantially cylindrical chamber with a substantially cylindrical susceptor mounted co-axially within the chamber, thereby defining an annular gap there between. A flexible, rectangular substrate is wrapped on the susceptor so as to cover the circumference of the susceptor. The flexible substrate is in direct thermal contact with the susceptor. The length of the substrate is substantially equal to the circumference of the susceptor, whereas the width of the substrate is substantially equal to the width of the susceptor. The unique ALCVP reactor is further provided in some embodiments with a stationary resistance heater that is mounted underneath the susceptor. Electrical energy to the heater is provided from heater supply power cables that pass through an axially mounted hollow shaft of the susceptor. In one embodiment an open end of the hollow susceptor shaft is encased within another rotary vacuum seal with fixed vacuum feed-through connectors, which establish electrical contact between the heater power supply cables and an external power source. Optionally, longitudinal optical heaters can be mounted on the external chamber walls to transmit radiation on to the substrate through transparent windows installed within the walls of the chamber. In both of these modes of substrate heating, internal and stationary non-contact temperature probes may be mounted within the susceptor cavity to monitor the temperature uniformity of the susceptor and also to control the susceptor temperature in a closed-loop fashion. Connections for non-contact temperature probes may be established through a fixed vacuum feed-though to facilitate closed-loop temperature control. In the embodiments described herein a substrate processing region is adapted to enclose the substrate during processing. A load-unload port opening to the substrate processing region is provided to transfer the substrate in to and out of the substrate processing region. Also a door is provided to load and unload the substrate and to close the load-unload port during processing. Also, the door may be adapted to provide a vacuum seal to the chamber in closed position. The substrate processing region is preferably interposed between the gas injection region and the susceptor that supports the substrate during processing. In this embodiment, the ALCVP reactor comprises at least one composite nozzle mounted within the circular chamber. The composite nozzle is mounted substantially parallel to the axis of rotation of the susceptor on the circumference of the chamber. The composite nozzle comprises one or more inner linear injectors mounted either within or in the vicinity of at least one outer exhaust. Each inner linear injector is provided with a plurality of apertures on one side that direct gas emanating from the apertures towards the substrate. The inner linear injector may be closed at one end with a gas inlet at the opposite end or the inner linear injector may be closed at both ends with a gas inlet in the middle. The inner linear injector is connected to a controlled and metered source of a gas, for example to a mass flow controller. The length of the inner linear injector mounted within the composite nozzle is substantially equal to the width of the susceptor. During the operation of the apparatus, the gas is directed from the inner linear injector towards the substrate so as to cover the width of the substrate while the outer exhaust port simultaneously collects the excess (non-chemisorbed or un-reacted) gas from the surface of the substrate. While the gas is being flown from the inner linear injector and simultaneously being collected through the outer exhaust port of the composite nozzle, the susceptor is rotated around its axis to ensure sequential coverage of the substrate by the gases. It is well-known that the gas flow on a rotating cylinder is deflected in the direction of rotation, to compensate for such a deflection, the alignment of the gas flow with respect to the susceptor surface (angle made by the direction of the gas flow from the inner linear injector with respect normal to the susceptor surface) can be changed by simply tilting the inner linear injector. The outer exhaust port of the composite injector is connected to a gate valve which in turn is connected to a throttle valve. The throttle valve is connected to a vacuum source e.g., a vacuum pump through a reactive gas/chemical vapor collection trap to collect the unused chemical precursor/gas injected into the ALCVP reactor.
In a preferred embodiment, the ALCVP reactor is provided with four composite nozzles mounted on the circumference of the chamber and substantially parallel to the axis of the chamber. The composite nozzles are positioned in the sequence such that the angular separation between the two adjacent composite nozzles is substantially same. Also, the chamber is provided with four rectangular flow partitioning plates extending in radial direction inward from the circumference of the chamber. Each flow partitioning plate is positioned in the space between two adjacent composite nozzles along the circumference of the chamber. Moreover, each flow partitioning plate is mounted on the bellows so that separation between its lower edge and the substrate surface can be adjusted as desired. During the operation of the ALCVP reactor, a first composite nozzle injects a first reactive gas A and also simultaneously collects excess or non-chemisorbed gas A from the substrate surface. A second composite nozzle injects an inert gas P on to the substrate to sweep off and collect non-chemisorbed (excess) reactive gas A along with the inert gas P from the substrate. A third composite nozzle injects a second reactive gas B and also collects non-chemisorbed second reactive gas B from the substrate. A fourth composite nozzle injects the inert gas P on to the substrate and simultaneously collects the reaction by-products of the atomic layer processing reaction between the first reactive gas A and the second reactive gas B in addition to the inert gas P. Continuous rotation of the substrate attached to the co-axially mounted susceptor while the four composite nozzles continuously operate subjects the substrate surface sequentially to a gaseous reaction process comprising the first reactive gas A, the inert gas P, the second reactive gas B and the inert gas P thereby completing one atomic layer chemical vapor processing sequence. Processing of the thin film of desired thickness is achieved by rotating the substrate through pre-determined number of rotations. The exhaust ports of the first and second composite injectors are connected to a common pipe which is connected to a first gate valve. The first gate valve is connected to a first throttle valve which is in turn connected to a vacuum pump through a first chemical condensation/collection trap. Similarly, the exhaust ports from the third and fourth composite injectors are connected to a common pipe which is connected to a second gate valve. The second gate valve is connected to a second throttle valve which is in turn connected to the vacuum pump through a second chemical condensation/collection trap.
In another embodiment, the ALCVP reactor is provided with four composite nozzles wherein the first composite nozzle and the third composite nozzle each comprise at least two inner linear injectors and each inner linear injector is connected to a distinct, controlled and metered reactive gas supply. The composite nozzles are positioned in the sequence such that the angular separation between the two adjacent composite nozzles is substantially same. The ALCVP reactor is also provided with four rectangular flow partitioning plates. The reactive gases being supplied to any one of the composite nozzles are selected such that do not react spontaneously with each other within the composite nozzle. However, these reactive gases collectively exhibit reactivity towards the reactive gases being supplied to the other composite nozzle. In this embodiment of the ALCVP reactor, the first composite nozzle employs first reactive gas A and third reactive gas C; the second composite nozzle employs the inert gas P; third composite nozzle employs reactive a second reactive gas B and the fourth reactive gas D and the fourth composite nozzle employs an inert gas P. In this particular configuration of the ALCVP reactor, first atomic layer chemical vapor processing sequence comprising the first reactive gas A, an inert gas P, the second reactive gas B and the inert gas P (for example: A, B, P and P) is carried out initially by rotating the susceptor to sequentially expose the substrate to all the required composite nozzles set in operation. The first atomic layer chemical vapor processing sequence is followed by a second atomic layer processing sequence comprising the third reactive gas C, inert gas P, the fourth reactive gas D and inert gas P, (for example: C, P, D and P) without removing the substrate from the ALCVP reactor. Alternatively, a thin film of variable composition comprising elements derived from all the reactive gases A, B, C and D can be processed. Also, composition of the film can be varied in-situ during processing by simply properly adjusting (or switching off, if desired) flows of one or more reactive gases selected from the group comprising A, B, C and D. Furthermore, an alternating double layer structure comprising (AB)m-(CD)n-(AB)o-(CD)p . . . (here, m, n, o and p are all integers ) can be processed by suitably switching the flows of the reactive gases A, B, C and D on and off while rotating the substrate though pre-determined number of rotations.
In yet another embodiment the ALCVP reactor is provided with four composite nozzles and four rectangular flow partitioning plates. The composite nozzles are positioned in the sequence such that the angular separation between the two adjacent composite nozzles is substantially same. The inner linear injector of the first composite nozzle is connected to the distinct and independently controlled supplies of reactive gases denoted by symbols A1, A2, and A3. The inner linear injector of the second composite nozzle is connected to an inert gas P. The inner linear injector of the third composite nozzle is connected to the distinct and independently controlled supplies of reactive gases denoted by symbols B1, B2, and B3. The reactive gases of first group A1, A2 and A3 are selected such that they do not react with each other spontaneously. So also the reactive gases of second group B1, B2 and B3 do not react with each other spontaneously. However, the reactive gases of the first group exhibit high reactivity towards the reactive gases of second group which is highly desirable to perform ALCVP type of processes. In this embodiment of the ALCVP reactor, a thin film of composition comprising all six elements A1, A2, A3, B1, B2, and B3 can be processed with varying degree of relative concentrations of all six elements.
In another embodiment the ALCVP reactor is provided with six composite nozzles, including two composite nozzles supplying inert gas, mounted substantially parallel to the chamber axis within a circular chamber. The ALCVP reactor is also provided with four flow partitioning plates.
In an alternative embodiment of the ALCVP reactor the cross section of the susceptor is a polygon, preferably an octagon with each face shaped as a trapezoid, mounted co-axially within a substantially circular atomic layer processing chamber. Furthermore, each trapezoid shaped face of the susceptor has provision to hold at least one substrate. Except for the geometry of the susceptor, the details of configuration of the ALCVP reactor are similar to those described in the preferred embodiment. During the operation of the ALCVP reactor, the susceptor with multiple individual substrates mounted on it, is rotated around its axis while the reactive and inert gases are flown from all the composite nozzles to ensure complete coverage of the substrates by the gases in a desired sequence. Processing of thin film of desired thickness is achieved by pre-determined number of rotations. Also, each face of the trapezoidal susceptor makes an acute angle with respect to the vertical axis of the chamber in order to facilitate holding of the substrate during susceptor rotation. For an inclined susceptor configuration, all the composite nozzles are also mounted inclined substantially at the same angle with respect to vertical, and thus substantially parallel to the surface of the susceptor.
In yet another embodiment of the present invention, at least two cylindrical shaped susceptors are mounted within a rectangular shaped chamber. A flexible metallic belt is employed as a substrate holder which is in direct thermal contact with the susceptor. At least one flexible substrate is mounted on the substrate holder. Each cylindrical susceptor is further provided with a stationary heater mounted concentrically underneath. Additionally, two longitudinal stationary heaters are provided within the space defined by the two cylindrical shaped susceptors to heat the substrate. Four composite nozzles are mounted on the periphery of the rectangular chamber and in the vicinity of the susceptor such that during the operation of the apparatus the inner linear injector, each having a plurality of apertures along one side and mounted within the composite nozzles, directs the gas towards the substrate to substantially cover the substrate width. Simultaneously the outer exhaust port of the composite nozzle collects the excess gas in the vicinity of the substrate subsequent to its impingement on the substrate while the substrate attached to the flexible metallic belt is set in motion by rotating at least one of the cylinders. The sequence of installation of four composite nozzles is first reactive gas A, inert gas P, second reactive gas B and inert gas P in the direction of the rotation of the substrate. The desired atomic layer chemical vapor processing sequence is performed by rotating the substrates while all the composite nozzles are operational. Alternatively, in this particular embodiment of the ALCVP reactor, the flexible metallic belt may be entirely replaced by a flexible substrate which can be in the form of a sleeve that can be directly rolled on to the cylindrical shaped susceptors. The desired film thickness can be processed by simply rotating the substrate through pre-determined number of rotations.
In configurations of the ALCVP reactor as described above the flexible substrate can be rolled and fitted onto the susceptor with a pair of ceramic end connectors that fit on to the susceptor. Also, the substrate may be mounted on the susceptor in the form of a sleeve. Alternatively the substrate can be held in position on to the susceptor by employing vacuum suction or an electrostatic chuck, or may be mounted in a recessed cavity. In the case of a polygonal susceptor, the substrate may be held in a recess in an inclined position on a facet of the polygonal susceptor. Additionally pins may be employed to hold the substrate in position. The substrate material is selected from, but is not limited to, metal-coated plastic, stainless steel, aluminum, molybdenum or suitable alloys of other metals, silicon, compound semiconductors e.g., silicon carbide, gallium arsenide, gallium nitride wafers, quartz or soda glass may be suitable substrate materials.
Accordingly, various configurations of a flexible substrate processing described herein are employed to process thin films of precisely controlled composition wherein the film composition can be dynamically changed in-situ through the film thickness as desired which is useful in the fabrication of graded band gap solar cells; multi-junction thin film solar cells; large area catalytic coatings with precisely tailored composition, precision interface engineering and multi-layer thin film optical coatings on large area substrates among a variety of other applications. Moreover substrate processing may be achieved at a significantly higher speed in atomic layer processing mode or at a significantly higher rate in chemical vapor processing mode. The rate of thin film processing in atomic layer mode largely depends-on the rate of rotation of the susceptor. The rate of thin film processing in high rate mode depends on rate of susceptor rotation, rate of reactive gas flows towards the substrate and the substrate temperature. Thus, the configurations of the apparatus of the present invention significantly accelerate the process of atomic layer processing on large-area flexible substrates, and also on multiple substrates within a small volume and small foot-print. The ALCVP apparatus of this invention in many embodiments is oriented such that the axis of rotation of the susceptor is parallel to the ground plane. However, it is important to note that since the substrate is wrapped and held on to the susceptor, any other suitable orientations of the axis of susceptor rotation will be equally effective for the operation of the apparatus. Operation of the apparatus and chemical processes for deposition of multi-layer, graded and multi-component thin film materials for photovoltaic solar cells and other applications are described in detail below with the help of various drawings.
The present invention relates to thin film processing—including at least deposition, etching and surface modification at a single atomic layer precision for a number of applications, including manufacturing of semiconductor devices, photovoltaic solar cells, displays and thin films, on large area flexible and planar substrates for applications such as catalytic electrodes, membranes and panels and so forth. The following descriptions are of various embodiments of the invention, and various modifications to the embodiments described will be apparent to those skilled in the art, and the patentable subject matter described and claimed herein may be applied to other embodiments. Thus the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and various features described herein.
The present invention provides atomic layer chemical vapor processing (ALCVP) apparatus configurations that can achieve coverage of flexible and also multiple planar substrates by reactive gases in a compact volume, small foot-print with flow stability and in a very short path length. In various embodiments a combination of rapid, repetitive and relative motion of a substrate with various gas injection schemes in the form of a steady flow of projecting gas jets achieves rapid and substantially complete surface coverage. It should thus be clearly apparent to an individual skilled in the art that such an apparatus is generic in nature and thus not limited by the reaction chemistry of the desired process to be performed on the substrate, for example, but not limited to, synthesis of a film, removal of the substrate material (etching) or modification of the chemical nature of the substrate. Hence, apar5atus in embodiments of the invention have a secondary purpose to process, using one or more embodiments described herein, a variety of thin films of metals, semiconductors and insulators and suitable combinations thereof with atomic level precision on one or more substrates under suitable process conditions. Furthermore, it should be noted that the operational range of processes for atomic layer chemical vapor processing is sufficiently wide with respect to operating parameters including, but not limited to, operating chamber pressure, gas flow rates and substrate temperature. Suitable operating pressure range can be from slightly below 760 Torr to a few hundred milli-Torr, whereas the reaction temperature is dependent upon particular vapor phase reaction chemistry. It is highly advisable in most cases to operate processes at minimum gas flow rates. However, the flow rates must be adequate to supply a sufficient quantity of reactive species to the substrate surface in order to obtain substantially complete, and if required, uniform surface coverage. To an individual skilled in the art adaptation of such a methodology of optimization of process parameters and also the tools required to achieve the same (for example mass flow controller, temperature controller, pressure controller, valve controls and closed loop control of the process parameters etc.), to control the process variables and to develop a desired process recipe is well-known.
The invention in several embodiments, including various apparatus designs and their operation, is described in detail in this section with the help of various schematic diagrams starting with existing apparatus as known to the inventor. A schematic of a multi-wafer, barrel-type CVD reactor system 10, in practice prior to the present invention, as an example of related art is shown in
It should be noted here that the inner inlet tubes 81a and 81b in the composite nozzle as shown in
In another embodiment the ALCVP reactor 240, the configuration of the first composite nozzle 80-1 and the third composite nozzle 80-3 is as shown in
It is specifically noted herein that in all the ALCVP reactor configurations described above (referring to
It should be noted herein that it may be necessary to heat all the connector pipes leading from the substrate processing chamber to the filter/trap through the gate valve and throttle valve in order to prevent condensation of the reactive gases (chemical precursors) in the exhaust. Both the chemical precursor traps, 428 and 446 respectively, are provided with a constant flow of coolant to help condense the reactive gases/chemical precursors. Such an arrangement has several potentially highly valuable benefits. First, the chemical precursor collection traps also help remove the solid particulates from the respective gas streams, which is highly beneficial for vacuum pump and its operation. Second, the unused reactive gases are locally injected and locally and separately collected and can be isolated in relatively purer state. This feature has significance in potential chemical re-use and also in enhancing the overall process and system operating efficiency and to substantially reduce the downstream effluent stream and its post-processing. Finally, by implementing closed-loop connection methodology between the input gas quantity and the collected quantity in the respective chemical precursor collection traps, the input reactive gas quantity can be optimized so as to reduce the collection in the chemical precursor traps to a minimum level and help run the overall process economically.
All the configurations of the atomic layer chemical vapor processing apparatus of the invention as described in detail can be operated in dual mode. The first mode of operation of the apparatus is as an atomic layer chemical vapor processing reactor to process the substrate at one atomic layer precision and the second mode of operation is a high-speed chemical vapor processing reactor.
In the first mode of reactor operation, to begin with, a substrate or multiple substrates, as the case may be, are firmly placed onto the susceptor and the ALCVP reactor door is closed to obtain a stable and constant internal pressure environment with the help of the O-ring seal. All four flow partitioning plates are lowered in to the chamber towards the susceptor such that the lower edges of all the flow partitioning plates are held in a fixed position in close proximity to the rotating susceptor. The distance between the lower edge of the flow partitioning plate and the susceptor surface can vary within the range of 1-5 mm depending upon the nature of the process gas, substrate temperature and reactor pressure. The ALCVP reactor is evacuated by opening the gate valve and the throttle valve with vacuum pump operational. The upper and lower inert gas purge and the susceptor cavity purge gas flows are initiated. Simultaneously, substrate rotation is initiated. Subsequent to attainment of the desired angular speed, which is thereafter maintained constant in a particular process step, the second and fourth composite nozzles both employing an inert gas P are activated. Simultaneously, the substrate is heated to the desired temperature and its temperature is maintained constant by supplying electrical energy to the embedded heater in closed loop fashion. During this step, the chamber pressure is also adjusted and held constant with the help of a closed loop arrangement between the throttle valve and the pressure sensor. Subsequently, the composite nozzle employing the first reactive gas A is activated and immediately thereafter the second composite nozzle that employs the second reactive gas B is activated. Activation of a nozzle for the present invention involves initiating the flow from an inner linear injector and simultaneously employing the outer exhaust port to evacuate the excess gas from the vicinity of the substrate. The substrate is thus continuously processed by exposing sequentially to the first reactive gas A, the inert gas P, the second reactive gas B and the inert gas P to process a single atomic layer on its surface. Subsequent to achieving the desired level of substrate processing, the first and third composite nozzles employing reactive gases are de-activated and the substrate is cooled in the flow of the inert gas flow from the second and fourth composite nozzles respectively, to a desired temperature while it is being continuously rotated. Subsequently, the angular speed of substrate rotation is gradually reduced and substrate rotation is fully stopped. The flow partitioning plates are retracted (moved outward) from the substrate surface. The gate valve is closed and the chamber is brought to a desired pressure level to transfer the substrate out of the chamber by opening chamber door to the substrate transfer port.
In a second mode of the reactor operation, subsequent to activation of all four composite nozzles, the second composite nozzle employing the inert gas P is de-activated by switching off flow of inert gas P and also turning off the valve in the outer exhaust port. Deactivation of the second composite nozzle results in termination of the process of formation of first monolayer (atomic layer) by the first reactive gas A and the overall process transforms into a high rate chemical vapor processing. Optionally, the fourth composite nozzle, employing the inert gas P, is also.deactivated. Also, in high rate chemical vapor processing mode all the flow partitioning plates are held in retracted position such that their lower edges are substantially away from the susceptor surface.
As an example of operating speed of the ALCVP reactor as described above, the substrate width can be 30 cm and the length can be approx. 100 cm. Such a substrate can be wrapped around a susceptor of approximate diameter=100 cm/π, which is approximately=32 cm. Assuming each monolayer of a thin film material is 0.2 nm in thickness and angular speed of susceptor rotation is 1000 rotations/min., the atomic layer deposition rate of 200 nm/min.; can be achieved on an area of one-third of a meter. An ALCVP reactor can achieve deposition thickness of 1.0 micron (1000 nm) in five minutes on a substrate size of one third of square meter! The invention is explained in further detail through its applications as described in the following examples:
Copper films can be deposited with one monolayer precision by employing cuprous halide with general formula CuX (X═F, Cl, Br and I) generated in-situ within the inner linear injector of the first composite nozzle, as described in the U.S. patent application Ser. No. 10/975,169; filed Oct. 27, 2004. The cuprous halide gas is subsequently combined with active hydrogen species (e.g., ionic species H+, free radicals H. and activated H2*) derived from H2 plasma. Alternately, CuX on the substrate surface can be combined with hydrogen free radicals (H.) obtained from a radical source connected to the inner linear injector of the second composite nozzle. For copper monolayer deposition process, the first and third composite nozzle each employs copper halide precursor while the second and fourth composite nozzles both employ species derived from hydrogen plasma or hydrogen free radicals to speed up the overall process.
The overall reaction is described as:
CuX+H+/H.→Cu+HX (2)
In the chemical process of copper halide reduction as described in equation (2), the active hydrogen species replace the inert gas P in a conventional four-step atomic layer deposition process. Alternate copper precursors that can be effectively used for this purpose are: Copper (II) hexafluoro-acetyl-acetonate [Cu(hfac)2], Copper (II) 2,2,6,6,-tetramethyl 3,5-heptanedionate [Cu(thd)2] among others.
Thin films of Copper Indium Diselenide can be deposited in ALD mode by employing one of the precursors of copper as described in example−1 above, which is combined with the appropriate precursor of indium such as halide of indium e.g., indium trichloride [InCl3] which can be generated in-situ within the linear injector [ref. U.S. patent application Ser. No. 10/975,169 filed Oct. 27, 2004], tri-methyl indium [(CH3)3In], di-methly indium chloride [(CH3)2In—Cl], indium hexa-fluoro-pentanedionate [C15H3F18O6In] among others. The precursors of indium are not limited to the ones listed above. The preferred selenium precursor is H2Se gas which can be generated in-situ from solid selenium and hydrogen as described in the U.S. patent application Ser. No. 10/975,169 filed Oct. 27, 2004. The overall chemical reaction for synthesis of copper indium diselenide thin films can be given as (for sake of simplicity the reaction is shown for chlorides and hydrogen selenide gas only):
CuCl+InCl3+2 H2Se→CuInSe2+4 HCl Eq. (2)
Nitrogen can be employed as an inert gas for purge in the ALD process. The configurations of ALCVP reactors that can be used for this purpose are shown in
Thin films of varying composition with thickness can be deposited in ALD mode by employing the ALCVP reactor configurations as described in
Thin films of ZnSe can be deposited by employing ZnCl2 as a zinc source and H2S or H2Se as source of sulfur and selenium respectively. ZnCl2 can be generated in-situ within an inner linear injector as described in the U.S. patent application Ser. No. 11/______ Alternatively, di-methyl zinc [(CH3)2Zn] can be employed as a zinc source.
CIGS is employed as an absorber layer and ZnSSe is a window layer in thin film photovoltaic solar cells. Bi-layer thin films of copper indium (gallium) selenide (CIGS)/ZnSSe thin films can be achieved by first depositing CIGS thin films as shown in example 3 above by employing an ALCVP reactor configuration as described in
The invention has been shown and described with reference to specific embodiments, which should be construed as examples only and do not limit the scope of practical applications of the invention. Therefore, any changes and modifications in technological processes, construction, materials, shapes and components are possible, provided these changes and modifications do not depart from the patent claims. For example, the composite nozzle, substantially linear in shape can be replaced by a set of multiple, individual nozzles that span the height of the susceptor. The susceptor in several embodiments has been described as a round drum or a rotatable element with multiple facets and a polygonal cross-section. In at least one embodiment described above, however, the system uses two rotating drums and a substrate or substrate carrier passes around both drums. The susceptor is thus a transport mechanism within the chamber, and many sorts of transport mechanisms are possible and probable within the scope of the invention. Alternately, a single-point nozzle projecting the flow on to the full height of the susceptor can be effectively employed to cover the substrate. Also, a large variety of chemical processes can be developed by employing the apparatus and methods described above. Furthermore, the process sequence can be suitably modified according to process chemistry and the desired product; however, all such modifications will fall within the scope of the invention. The operation of such a reactor can be modulated over a wide range of process parameters such as, gas flow rates, substrate temperature, substrate rotation speed and chamber pressure. In addition to deposition, the invention is equally applicable to other broad areas of processing such as etching or removal of materials, stripping of photoresist, post-ash or post-etch cleaning of resides in microstructures, removing deposits on the inner surfaces of the processing chamber and so on. It thus encompasses a broad area of substrate processing and is referred to by the inventor as Atomic Layer Chemical Vapor Processing, “ALCVP” and the processing chamber is termed the ALCVP reactor. Moreover, the configurations of the invention as described are not restricted to a particular chemical process and a wide range of chemistries can be effectively performed within its scope. The substrate shape need not be necessarily restricted to round or rectangular in shape and may have a square, polygonal or any other shape. Also various combinations and arrangements of the composite nozzles different from those shown and described are possible. Moreover, the susceptor, apart from being circular and octagonal in cross section, can be polygonal in shape. In the case of a composite nozzle, the inner linear injectors are not necessarily cylindrical tubes and may have a conical or any other shape. Such apparatus and methods of substrate processing are taught in sufficient and enabling detail.
Moreover, in all the ALCVP reactor configurations described above, the susceptor can be supplied with an electromagnetic source of energy e.g., radio-frequency excitation and can also be biased appropriately to modulate the properties of the thin film being processed and also the nature of the chemical reactions taking place on the surface of the substrate. Alternatively, at least one of the inner linear injectors can be connected to a plasma source or one of the linear injectors may be also connected to a source of free radicals to facilitate thin film processing reactions at lower temperatures.
Also, in all the atomic layer chemical vapor processing apparatus configurations described above, the mode of operation of the apparatus can be switched in-situ from discrete atomic layer processing to high-rate chemical vapor processing (deposition, etching or surface modification) mode of operation. During the atomic layer chemical vapor processing mode, all the rectangular flow partitioning plates are lowered towards the substrate to help break the boundary layer being formed on the surface of the substrate while all the composite nozzles, employing reactive gases as well as inert gases, are set in operation. Whereas, the high-rate mode of operation can be realized by either switching off the inert gas flow towards the substrate and/or by moving all flow partitioning plates away from the substrate surface.
Furthermore, it is quite important to note that in all the configurations of the ALCVP apparatus of the invention reactive gases are injected locally and are also collected locally and separately. In the downstream piping arrangement, the exhaust arm of the first composite injector and the exhaust arm of second composite injector are both connected to a Y (or a T) shaped connector which is in turn connected to a vacuum pump through a throttle valve and a chemical precursor collection/condensation trap. Similar downstream piping arrangement is employed for the third and fourth composite injectors respectively. Such piping arrangement in the exhaust section of the ALCVP apparatus averts downstream mixing of highly reactive chemical precursors and significantly helps in the recovery of the unused portion of the precursors in relatively pure form for their potential reuse to realize significant operational savings. Also, such an arrangement also greatly reduces the quantity of downstream effluents, extent of waste remediation and costs associated with it. In addition, the various reactor configurations described herein also help optimize the chemical precursor consumption. A combination of these factors help substantially increase the overall process speed and also the operating efficiency of the apparatus described in the invention.
This application claims the benefit of the U.S. provisional application Ser. No. 60/656,772 filed Feb. 26, 2005 which is incorporated by reference herein.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US06/07715 | 3/3/2006 | WO | 00 | 2/2/2009 |
