Patent Application
20100180913
The present invention is related to the field of semiconductor processing equipment and methods, and provides, in particular, methods and apparatus for in-situ removal of undesired deposits in the interiors of reactor chambers, for example, on chamber walls and elsewhere.
Halide (or hydride) vapor phase epitaxy (HVPE) is an epitaxial process for rapidly growing compound semiconductor materials, in particular, Group III-V compound semiconductors such as GaN. Because of the high growth rates achieved by HVPE, it is ideal for production of thick, free-standing GaN layers. HVPE processes grow epitaxial GaN by reacting a Ga-containing precursor gas and an N-containing precursor gas at the surface of a heated substrate (e.g., usually 800-1200° C.). Most HVPE processes produce a GaCl precursor gas by passing HCl over heated, liquid Ga held in the reactor chamber. The N-containing precursor gas is usually NH3. In some HVPE processes, the Ga-containing precursor is GaCl3 vapor introduced into the reactor chamber from an external source.
However, during HVPE processes, material can grow or deposit, not only on the substrate, but also on undesired locations throughout the reactor chamber, e.g. on the reactor walls, on and around the susceptor, and elsewhere, and cause reduced throughput, increased costs, and even reactor damage. For example, undesired deposits, wherever they are located in the reactor chamber, can release particles, flakes, and so forth, which, if they lodge on the substrate, can render it undesirable, or even useless, for its intended purposes. Undesired deposits on and around a rotating susceptor can increase friction or even cause adhesion with stationary structures. Undesired deposits on the chamber walls can act as thermal insulators so that the heating/cooling times of the chamber are extended, thus reducing reactor throughput. In the case of quartz reactor chambers heated by IR radiation, undesired deposits on chamber walls can cause the quartz chamber itself to de-vitrify. Under typical operating conditions the IR radiation penetrates through the walls of the reactor chamber for heating of internal reactor components. However, the build up of undesirable deposits on the reactor walls increases IR absorption thereby increasing the wall temperature sufficiently for devitrification to occur.
In addition, it is often advantageous to perform a reactor chamber clean prior to each growth run executed in the system. Ensuring the chamber is clean and free of contaminates not only improves wafer quality but also resets the system into a known state from which all runs can be initiated. Consequently, the reset reactor state results in an increased repeatability of growth process from run to run, ensuing greater growth stability. The methods and systems of the invention outlined herein ensure that the duration of the cleaning processes is at a minimum, therefore improving material quality whilst minimizing the impact on wafer through put.
Therefore, reactor chambers, especially those used for HVPE processes must be periodically cleaned. Wet cleaning is one known reactor chamber cleaning method in which the chamber is exposed to cleaning solutions, e.g., strong acids, which dissolve the undesired deposits. Wet methods have disadvantages including time consuming disassembly and reassembly of the reactor subsystem, residual contamination left by cleaning solutions, and so forth. To remedy these disadvantages, dry cleaning methods have been developed in which undesired deposits are removed from the reactor chamber in situ. Deposits are often removed by converting them into a gas using reactive plasmas generated in the reactor chamber, or reactive gases introduced into the chamber, and the like.
In more detail, reactive gases used in dry cleaning processes are selected to lead to gas phase products upon reacting with the undesired deposits. In many cases, reactor chambers are heated to promote dissolution of the undesired deposits. Reactive gases can be introduced into a reactor chamber continuously, quasi-statically, and according to other known methods. In one known continuous method, fresh reactive gases are flowed continuously into the chamber and spent reactive gases along with reaction products of the undesired deposits are continuously exhausted from the chamber. See, e.g., U.S. Pat. No. 4,498,953, which is included herein by reference in its entirety for all purposes. In one known quasi-static method, cleaning proceeds by one or more cycles; in each cycle, an amount of fresh reactive gases are first introduced into the chamber, then the gases are retained statically in the chamber to permit reaction with undesired deposits; then after a period of time, the spent reactive gases along with reaction products of the undesired deposits are exhausted from the chamber. See, e.g., U.S. Pat. No. 6,620,256, which is included herein by reference in its entirety for all purposes.
Generally, processes for cleaning the reactor chamber are arranged and performed separately from growth processes conducted in the chamber. For example, cleaning is performed after growth is complete. However, certain growth processes proceed in separate steps, and reactor cleaning can be performed between the separate steps. U.S. Pat. No. 6,290,774, which is included herein by reference in its entirety for all purposes, describes a process for growing relaxed GaN layers on substrates in several separate steps, where in each step, a thin GaN layer is grown on the substrate at higher growth temperatures, and then the substrate is cooled to lower ambient temperatures to induce and relax thermal stresses. This patent further describes conducting chamber cleaning between the separate steps, that is, the chamber is cleaned after the substrate has been cooled in the previous step and before it is heated to growth temperature in the subsequent step.
However, it has been found that the known dry cleaning methods are not suitable for high-throughput HVPE material growth. Generally, the known methods are, by themselves, too inefficient, and also are too disruptive of the primary HVPE growth process. What are needed are more efficient dry cleaning methods that can be more tightly integrated into a primary HVPE process. With such dry cleaning processes, high-throughput production of thick layers of, in particular, Group III-V materials such as GaN, could be performed in reactor chambers maintained sufficiently free of undesired deposits so that the materials produced are of suitable qualities.
The present invention provides methods of chemical vapor deposition (CVD) and related reactor chamber subsystems, suited to the provided methods, by which semiconductor materials can be grown at high volume, with increased quality and to an increased thickness. Specifically, the methods allow prolonged periods of material growth without deterioration of material quality due to build-up of undesired deposits on the reactor chamber walls and on its internal components. When it is necessary to ameliorate undesired deposits, the methods rapidly cycle a reactor chamber from growth mode to an in situ cleaning mode, and then, when the undesired deposits have been sufficiently ameliorated, back to growth mode. The subsystems of the invention allow cycling between growth modes and cleaning modes to be rapid and efficient. It is believed that rapid cycling of the reactor chamber between growth processes and in-situ cleaning processes is not possible in the prior art.
In particular, during the growth mode, the invention preferably automatically, senses when a reactor chamber requires cleaning, and also, during the cleaning mode, senses when a reactor chamber is sufficiently clean. Cleaning is generally carried out at higher temperatures. Preferred subsystems provide the necessary sensors. Preferred cleaning sensors monitor the composition of gases exhausted from the reactor during the cleaning mode. Using the latter sensor, cleaning can be determined to be sufficiently complete when the level of products of the cleaning reaction is sufficiently low, e.g., at trace levels.
Generally, a working substrate is removed from the reactor chamber during cleaning to avoid chemical damage by the cleaning reagents, and further its removal/replacement generally is carried out at lower temperatures to avoid thermal damage. Preferred reactor chamber subsystems have means, e.g., controllable load lock, controllable robot arm, wafer pick-up tool and automatic control system, to rapidly perform, the essentially, mechanical substrate transfer. In preferred embodiments the controllable robot arm is capable of further increasing the rate of rapid cycling between reactor growth/clean modes and hence reactor throughput by permitting the loading and unloading of a substrate at elevated temperatures without incurring a deterioration in wafer surface quality.
Preferably a load lock opens into a load chamber, intermediate chamber, or the like, having a controlled atmosphere so that the substrate, when removed from the reactor chamber, can be held in controlled conditions out of contact with the ambient atmosphere.
In some (but not all) embodiments, cycling between growth and cleaning modes can require significant temperature decreases or increases. A preferred reactor chamber, therefore, has low thermal mass, such as reactor chambers made of, e.g., quartz and heated by infrared (IR) radiation, so that such temperature changes can be rapidly carried out. Further, preferred substrates are selected so that these temperature changes can be minimized. One class of substrates that is relatively resistant to thermal stresses comprises materials with sufficiently matched coefficients of thermal expansion (CTE) to a particular target growth material.
In preferred embodiments, the invention is applied to the growth of Group III-V semiconductor compounds, and in particular to Group III-nitride compounds such as GaN, by halide (or hydride) vapor phase epitaxy (HVPE). HVPE allows rapid growth of thick layers of Group III-nitride compounds, but such rapid growth can lead to accumulation of undesired deposits on the reactor walls and on its internal components, e.g., a growth wafer or susceptor. Accordingly, use of this invention in its preferred embodiments can provide the ability to grow very thick layers of Group III-nitride compounds without being limited by deterioration of reactor chamber cleanliness and without having to expose the working substrate to the ambient atmosphere.
For example, for Ga—V compounds (e.g., GaN or GaAs) during growth mode, reagent gases comprising a Ga-containing compound and a N-containing compound are introduced into the heated reactor chamber, and react to deposit a Ga-containing material. Preferably, the Ga-containing reagent gas comprises a Ga chloride introduced into the chamber from a source exterior to the chamber. When undesired deposits have accumulated to an unacceptable level, the reactor chamber subsystem switches (preferably automatically) to the cleaning mode. During cleaning mode, cleaning gases comprising a halogen or halogen compound are introduced into the heated reactor chamber and react with the undesired deposits to form gaseous reaction products. The working substrate with Ga-containing material grown thereon is removed during cleaning. The flow of cleaning gases is stopped once the exhaust gas sensor indicates that the gases exhausting from the reactor chamber comprise little or no Ga-containing compounds. Then, the reactor chamber subsystem switches (preferably automatically) back to the growth mode, and the growth-cleaning cycle is repeated until a desired amount of Ga-containing material has been deposited on the substrate. These methods are preferably carried out with reactor chamber subsystems having the above-described preferred features.
In more detail, the present invention provides preferred embodiments with a method for growing a selected amount of a semiconductor material on a substrate in a reactor chamber which includes the step of growing the semiconductor material on the substrate by a chemical vapor deposition (CVD) process; and removing undesired deposits within the reactor chamber by an in situ cleaning process, wherein the steps of growing and removing are repeated in a manner so that the selected amount of material is grown on the substrate while the amount of undesired deposits in the reactor chamber is maintained within an acceptable range.
In further preferred embodiments, the CVD process can be a halide vapor phase epitaxy process that grows on the substrate one or more compounds of one or more Group III elements; the in situ cleaning process can include converting undesired deposits to gaseous products which are exhausted from the reactor chamber; the acceptable range of accumulation is such that the material grown on the substrate has a quality sufficient for its intended use or is substantially free of contamination arising from the undesired deposits; the growth step, or the removal step, or both steps, can be performed for periods of time selected so that the amount of undesired deposits is maintained within an acceptable range; the amount of undesired deposits can be detected automatically, and the in situ cleaning process is performed in dependence on the automatically-detected amount undesired deposit so that the amount of undesired deposits is maintained within an acceptable range; and the substrate can be transferred from the reactor chamber during the in situ cleaning process with reactor chamber temperature during substrate transfer set within a replacement/removal temperature range such that thermal damage to the substrate is not likely.
In further preferred embodiments, the present invention provides a method for in situ cleaning of deposits from the interior of a reactor chamber, which is useful in semiconductor equipment, that includes exposing the interior of the reactor chamber to a gas which reacts with the undesired deposits to form gaseous reaction products, detecting automatically levels of the gaseous reaction products, optionally by performing a spectral measurement, and continuing the gas exposure until the automatically detected levels of reaction products indicate that the amount of undesired deposits is within an acceptable range.
In further preferred embodiments, the levels of gaseous reaction products can be detected in gases flowing in the reactor-chamber exhaust after having flowed through the body of the reactor chamber; the undesired deposits can comprise one or more Group III-V compounds, the cleaning gases can comprise one or more halogen compounds; the reactor chamber can be heated during the gas exposure to sufficient temperatures (which can be below, or about, or above the temperatures that prevailed in the chamber during formation of the undesired deposits.
In further preferred embodiments, the present invention provides processing equipment for growing a selected amount of a semiconductor material on a substrate that includes a reactor subsystem with a reactor chamber, the subsystem being directed by control signals to carry out various semiconductor process, a gas sensor for generating signals responsive to the composition of gases discharged from the chamber, and an automatic controller for generating control signals to direct the reactor subsystem, the control signals being generated, at least in part, in dependence on the gas-sensor signals.
In further preferred embodiments, the control signals include cleaning control signals that carry out an in situ process for cleaning undesired deposits from within the reactor chamber, and wherein the in situ cleaning process is continued until the gas-sensor signals indicate that the remaining amount of undesired deposits within the reactor chamber is within an acceptable range; the in situ cleaning process can includes particular steps of exposing the reactor chamber to one or more cleaning gases that react with the undesired deposits within the reactor chamber to form gaseous reaction products, and discharging the reaction products from the reactor chamber.
In further preferred embodiments, the control signals include growth control signals that carry out CVD processes for growing semiconductor material on the substrate within the chamber, and wherein the controller repetitively generates the growth-control signals and the cleaning-control signals in a manner so that the selected amount of material is grown on the substrate while the amount of undesired deposits in the reactor chamber is maintained within an acceptable range; preferably, the equipment can also include a deposit sensor for generating signals responsive to undesired deposits within the reactor chamber, and wherein the CVD process is continued until the deposit-sensor signals indicate that the reactor chamber should be cleaned; the CVD process can heat the reactor to a growth temperature range and flow through the reactor chamber one or more gases that react to deposit the material on the substrate; the precursor gases comprise halogen compounds of a Group III element, and the growth temperature can range from about 800° C. to about 1150° C.
In further preferred embodiments, the equipment also includes a substrate-transfer means, optionally a robot arm, directed by transfer control signals for performing processes for transferring a substrate into or out of the reactor chamber, the substrate being transferred out of the reactor chamber prior to the in situ cleaning process and transferred back into the reactor chamber subsequent to the in situ cleaning process; the transfer process that is performed preferably includes maintaining the reactor at a replacement/removal temperature during the substrate transfer, the replacement/removal temperature being such that thermal damage to the substrate during transfer is unlikely, and for example can be from about 600° C. to about 750° C.
Headings are used herein for clarity only and without any intended limitation. A number of references are cited herein, the entire disclosures of which are incorporated herein, in their entirety, by reference for all purposes. Further, none of the cited references, regardless of how characterized above, is admitted as prior to the invention of the subject matter claimed herein.
The present invention may be understood more fully by reference to the following detailed description of the preferred embodiment of the present invention, illustrative examples of specific embodiments of the invention and the appended figure in which:
The present invention provides efficient dry-cleaning methods for reactor chambers used for vapor-phase material growth processes, in particular for reactor chambers used for CVD (chemical vapor deposition), PECVD (plasma enhanced CVD), MBE (molecular beam epitaxy), and so forth, for the growth of semiconductor materials. The invention also provides methods for high-throughput, vapor-phase material growth that incorporates and integrates the provided cleaning methods so that the reactor chambers are kept sufficiently free of undesired deposits. The invention also includes apparatus for epitaxial growth that includes particular features directed to efficiently perform the provided methods.
Generally, the invention is applicable to vapor-phase growth processes for many types of materials, as will be apparent to those of skill in the art. In preferred embodiments, the materials of interest are “semiconductor materials”, a term which is used herein to refer to both active semiconductor materials (e.g., Si, SiGe, GaN, and so forth) as well as to additional materials used in component fabrication (e.g., SiO2, W, and so forth). Semiconductor materials preferably include Group III-V compound materials, particularly Group III-nitride compound materials, and more preferably to pure and mixed nitrides of Ga, Al, and In.
The term “substrate” (or “wafer”) is used to refer to the base or foundation substance on which material is deposited, and also to the base or foundation material on which one or more layers have been grown. Substrates can have a homogenous or a heterogeneous composition, e.g., can include a plurality of layers of different materials. The semiconductor materials (or material generally) grown on a substrate can be similarly homogenous or heterogeneous.
Chambers are kept “sufficiently clean” if materials grown therein are sufficiently free of contamination so that they can be fabricated into electronic, optical, or opto-electronic components; a sufficiently clean reactor chamber has an “acceptable” level of undesired deposits. A level of deposits is “unacceptable”, and a reactor with such deposits is not sufficiently clean, if materials grown in a reactor chambers with that level of deposits are not suitable for their intended purpose. Whether or not a particular level of deposits is acceptable or unacceptable is controlled by several factors including required material quality, growth process, reactor chamber geometry, flow conditions in the reactor, etc., and can be determined by testing material quality as a function of level of undesired deposits while keeping other factors constant.
The following description is often focused on embodiments suitable for the growth of particular semiconductor materials, especially Group III-nitride materials such as GaN. However, this descriptive focus is only for conciseness and clarity. It should be understood that it does not limit the invention to the particular embodiments focused on.
By way of brief background, Group III-nitride compounds (either pure of mixed nitrides) are usually grown using either MOCVD (metal-organic CVD) or HVPE (hydride/halide vapor phase epitaxy). In MOCVD, the Group III-precursor, a metal-organic compound, and nitrogen precursor, usually NH3, are introduced from outside the reactor chamber and flowed over a heated substrate supported by a susceptor where the Group III-nitride compound grows. In HVPE, the Group III-precursor is a metal chloride, which can either be introduced from outside the chamber, or can be produced inside the chamber by flowing HCl over the heated Group III-metal. The nitrogen precursor is again usually NH3, and the substrate is heated to about 800-1100° C. HCl is a suitable etchant gas for both MOCVD and HVPE, and during cleaning, the reactor chamber is heated to near or beyond the Group III-nitride growth temperatures. H2 is a gas with which to purge lingering etchant gases from the reactor chamber prior to resuming growth.
During cleaning, a single etchant gas can be used, or a combination of etchant gases can be used, or different etchant gases can be used in succession. Etchant gases are chosen for their ability to react with undesired deposits under conditions compatible with the underlying growth process to form gas-phase products that can be readily exhausted from the chamber. In particular, etchant gases should not leave residues that can contaminate the growth process or lead to damage of the reactor chamber itself. For example, etchant gases can be selected to (thermodynamically) force the growth process to run backwards leading to dissolution of undesired deposits. The chamber may or may not need to be heated during etchant gas flow. In addition to HCl, preferred for Group III-nitride growth process, suitable etchant gases are often halogen containing, e.g. elemental halogens (e.g., F2 and Cl2) and compounds of halogens with hydrogen, other halogens, inert gases, rare gases, and the like (e.g., HCl; BCl3; SiCl4; ClF3; NF3; etc.). Etchant gases may be used in their native state or activated by passage through a plasma.
Growth steps 103 and 119 are repeated until material growth is complete 121, and cleaning step 111 is repeated with sufficient frequency and is continued for a sufficient duration so that undesired deposits are limited to acceptable levels throughout all the growth steps. Accordingly, during the process, it must be determined when undesired deposits have sufficiently accumulated so that growth should be interrupted and cleaning commenced; and it must also be determined when undesired deposits have been sufficiently dissolved so that cleaning can be terminated and growth resumed. One or both of these decisions can be made by an operator. For example, an operator can monitor (e.g., by visual inspection) the reactor chamber during growth, decide when undesired deposits have accumulated to an extent such that the chamber should be cleaned, and then trigger an interruption of growth and commencement of cleaning. The operator can then monitor (e.g., again by visual inspection) the reactor chamber during cleaning, decide when the chamber is sufficiently free of undesired deposits, and then trigger termination of etchant gas flow, purging of etchant gases from the chamber, and resumption of growth.
In preferred embodiments one or both of these decisions can be made automatically so that operator inattention or inefficiency need not delay an ongoing high-throughput growth process. In one embodiment, one or both of these decisions can be made according to elapsed time. For example, from experimentation and experience with a particular reactor chamber and a particular growth process performed with substantially fixed parameters (e.g., pressures, temperatures, flow rates, and the like), an elapsed time for the accumulation of undesired deposits to unacceptable levels can be determined. Similarly, an elapsed time can be determined for the dissolution of an acceptable level of undesired deposits from a particular reaction chamber in which a particular etchant gas is flowed at known flow rates, temperatures, and the like. Then, growth steps 103 and 119 can be performed for a time duration determined in dependence on the elapsed accumulation time, and cleaning step 111 can similarly be performed for a time duration determined in dependence on the elapsed dissolution time.
In more preferred embodiments, one or both of these decisions, when to interrupt growth and commence cleaning 107 and when to interrupt cleaning and resume growth 115, are made automatically in dependence on sensor signal inputs. For example, the decision when to interrupt growth 107 can be dependent on inputs from deposition sensors responsive to the amount of undesired deposits that have accumulated within a reactor chamber. When the deposition sensor signals indicate an unacceptable level of undesired deposits is imminent, growth can be automatically interrupted and cleaning can be triggered. The decision when to interrupt cleaning can also be made in dependence on inputs from the same deposit sensor. When the deposition sensor signals indicate that sufficiently little undesired deposit remains in the reactor, cleaning can be automatically interrupted and growth resumed.
However, the decision when to interrupt cleaning is preferably made in dependence on the composition of exhaust gases from the reactor chamber during cleaning, in particular, in dependence on the amount of products of the reaction between the etchant gases and the undesired deposits found in the exhaust. The complete composition need not be measured; it can be sufficient to measure markers, fingerprints, signatures, and the like that distinguish reaction products from other components of the exhaust. Also, such markers need not be continuously monitored; intermittent sampling can be sufficient. Such markers can include spectral characteristics of the exhaust. When the measured or sampled markers indicate that the level of deposit reaction products in the exhaust is sufficiently low, cleaning can be automatically interrupted and growth resumed. The decisions when to interrupt growth or cleaning steps can be made in other ways that will be apparent to those of routine skill in the art.
As discussed, substrates can be chemically damaged by etchant gases during cleaning, and must therefore be protected from exposure to these gases. In most embodiments, substrates are removed from the reactor chamber 109 prior to commencing cleaning and replaced in the reactor chamber 117 prior to resuming growth. These steps are essentially mechanical and require opening and closing the reactor and manipulating the substrate. Although these steps can be performed by manual operator action, such manual performance is not preferred during high-throughput growth processes. Inattention or inefficiencies of even well trained operators can introduce delays. Therefore, reactor chamber subsystems used in this invention preferably includes automatically controllable devices that perform substrate removal and replacement in response to control signals. Automatic implementation of these steps is further described with reference to
Chamber cleaning can be performed with the chamber in place, or the chamber could be removed and placed in a cleaning subsystem. Next, materials, e.g., growth substrates, sensitive to the etchant gas to be used are removed 109 from the chamber. Removed materials are replaced 117 when the cleaning is complete. The etchant gas (or gases) are now admitted 111 to the chamber as a continuous flow or as intermittent pulses. The progress of cleaning is monitored 113, preferable as above, by sensing gases exhausting from the chamber for markers of reaction products of the etchant gases and the undesired deposits. The markers can be spectroscopic characteristics of the exhaust gases. When cleaning is determined to be complete, e.g., by the level of reaction products in the exhaust gases falling to a sufficiently low level, the cleaning process ends 123. It will be apparent to one of skill in the art that the cleaning methods of this invention and their steps can be alternatively incorporated and integrated into other growth processes, or even into other processes, that are performed in a rector chamber, or can also be arranged differently in different standalone embodiments. These alternatives are within the scope of this invention.
The methods of this invention are advantageously implemented in connection with reactor chambers, reactor-chamber subsystems (and/or growth/deposition systems) with certain preferred features enabling automation of one or more steps of the cleaning methods. Preferred features include: sensors of reactor-chamber-exhaust gas composition; sensors of undesired deposits; controllable (e.g., robot) mechanisms for transfer of substrates to and from reactor chambers; selectable gas species for wafer pick-up components; controllable doors between the reactor chamber and its exterior; controllable etchant gas inlets; a load or intermediate chamber where substrates removed from a reactor chamber can be held out of contact with the ambient atmosphere; automatic control systems for receiving sensor signals and outputting control signals; and the like.
Specific embodiments of such reactor chambers and associated subsystems with the above general features and directed to high-throughput growth of Group III-V-compound containing materials, e.g., GaN semiconductor material, are described in U.S. provisional patent applications Nos. 60/866,910 filed Nov. 22, 2006; 60/866,965 filed Nov. 22, 2006; 60/866,928 filed Nov. 22, 2006; 60/866,923 filed Nov. 22, 2006; 60/866,953 filed Nov. 22, 2006; 60/866,981 filed Nov. 22, 2006, all of which are incorporated herein by reference in their entireties for all purposes. The described embodiments use HVPE processes with external sources of a Group III-chloride precursor (e.g., GaCl3) and include features which slow the accumulation of undesired deposits, e.g., reactor chambers walls kept at temperatures considerably below deposition temperature.
The illustrated reactor chamber also includes specific features useful for the methods of this invention. Certain preferred specific features are directed to the controllable admission of etchant gases. Etchant gases can be admitted into the chamber through separate inlets 227, or alternatively, etchant gases can be admitted through inlets used also for precursor gases. However admitted, actual admission of etchant gases is preferably controllable, e.g., by controllable valves 229 (or mass flow controllers, or the like).
Further preferred features are directed to monitoring the amount or extent of undesired deposits so that growth can be interrupted automatically and without operator delay upon accumulation of excessive deposits. Accumulation of undesired deposits can be optically monitored, since such deposits generally reflect light, or absorb light, or both, and since levels of reflectance or absorption generally depend to at least some degree on the level of undesired deposits. Accordingly, exemplary optical sensors 237a and 237b are arranged to measure either light reflectance at the quartz walls of reaction chamber 11, or light transmission through the walls and across the chamber, or both, and to provide signals 241 to the control systems 239. In further embodiments, details of reflection and absorption such as reflectance at selected angles or absorption at selected frequencies, can be measured in order to improve sensitivity and selectivity of deposit monitoring. Also, accumulation of undesired deposits on selected components internal to the reactor chamber can be optically monitored using light focused on the selected components.
Alternatively, the presence and amount of undesired deposits can be indirectly sensed by their effects on the reactor chamber and its internal components. For example, undesired deposits can be sensed by measuring the increased temperature of the walls of reactor chamber 11, because such deposits on the walls of reactor chamber 11 absorb IR radiation from lamps 247 and thereby increase the wall temperature. Also undesired deposits can be sensed by measuring changes in the operating characteristics of a reactor chamber. Undesired deposits on a rotating susceptor and its supports can increase friction or change other rotational characteristics of the susceptor. Undesired deposits can partially occlude gas inlet ports, exhaust ports, and the like, and measurably change the characteristics of gas flow through these ports.
Further preferred features are directed to monitoring the progress of reactor cleaning so that cleaning can be interrupted automatically and without operator delay when the reactor chamber is sufficiently clean. Cleaning can be monitored by the same means used to monitor accumulation of unwanted deposits. For example, cleaning can be interrupted when signals from the above-described optical sensors indicate that little or no undesired deposits remain in the reactor chamber. Preferably, however, cleaning is monitored by sensing or sampling the composition of gases exhausted from the reactor chamber during the cleaning step. These gases include products of the reaction between the etchant gases and the undesired deposits, and it is believed that, when cleaning nears completion, the concentration of these reaction products will decrease towards trace amounts or even to zero. Accordingly,
Suitable chemical analyzers can be based on known chemical analysis technologies, in particular, on analysis of various types of spectra. For example, infrared (IR) spectra of gases passing through the exhaust line can be used to determine the concentration of selected species in the exhaust, since such spectra reveal the distinctive vibration signatures of chemical species in the exhaust. For example, reaction products of undesired deposits including GaN (or other Group III-nitride) with etchant gases including HCl typically contain various Ga-chloride (or other Group III-chloride) species which have distinctive vibration signatures detectable in an IR spectrum. Accordingly, analyzer 235 can include an IR spectrometer such as a Fourier Transform IR (FTIR) spectrometer. In addition, UV absorption spectral techniques could be utilized as supplementary optical techniques. Further, mass spectra can distinctively identify reaction products. Accordingly, analyzer 235 can include a mass spectrometer such as a time-of-flight spectrometer, a quadrupole spectrometer, or other type of mass spectrometer.
Further preferred features are directed to performing, automatically and with little or no operator attention, substrate (more generally, work item) removal prior to chamber cleaning and replacement subsequent to chamber cleaning One such feature is robot arm 231, or similar, which is controlled by controller 231a so as to cause the arm to execute a sequence of actions which physically remove a substrate from the interior of a reactor chamber to an exterior location, and also physically replace the substrate from the exterior location back into the interior of reactor chamber. The robot arm can employ front or backside wafer pick-up techniques. In preferred embodiments and for high temperature applications, the robot is fitted with a pick-up wand operating on the Bernoulli principle (Bernoulli wand 233). See, e.g., U.S. Pat. No. 5,080,549, which is incorporated herein by reference in its entirety for all purposes. A Bernoulli wand utilizes downward jets of gas towards the substrate to create a region of low pressure above the substrate leading to a pressure difference across the substrate that lifts and holds a typically hot substrate without contacting the substrate. Bernoulli wands can reduce substrate contamination and temperature gradients in comparison to pick-up devices that physically contact the substrate.
Additional preferred substrate removal/replacement features cooperate with the robot arm and Bernoulli wand to provide automatic access to the reactor chamber, e.g., load lock 215, and handling of the substrate when it is removed from the reactor, e.g., intermediate transfer (or load) chamber 213 and associated components. Load lock 215 includes a door that can be automatically closed to seal the reactor chamber during growth and cleaning, and also can be automatically opened to allow the robot arm access to a substrate within the reactor chamber. When interior to the reactor chamber during growth, substrates are usually supported on a substrate holder, such as susceptor 217. When exterior to the reactor during cleaning, substrates can be supported on a substrate holder, e.g., substrate holder 245 within the load chamber or holder 251 without the load chamber, or held by the robot arm. Robot arm 231 can access substrate holder 251 through automatically controllable rear lock door 216 between load chamber 213 and exterior 249.
The load chamber and associated components can perform further functions useful to improving substrate transfer speed. See, e.g., U.S. Pat. No. 6,073,366, which is incorporated herein by reference in its entirety for all purposes. For example, the load chamber and load lock door can then function similarly to an air lock. Prior to opening the load lock door leading to the reactor chamber, the load chamber atmosphere can be controlled to have a pressure substantially equal to the pressure of the reactor chamber atmosphere, or to have a composition that does not react with the reactor chamber atmosphere and the reactor chamber contents (e.g., is inert), or to be otherwise compatible with the reactor chamber atmosphere.
Similarly, prior to opening the rear lock door, the load chamber atmosphere can be controlled to have substantially atmospheric pressure, to have no toxic components, or the like. Although, preferably, a substrate is retained in the load chamber when removed from the reactor chamber, and the load chamber has an atmosphere controlled so as not to react with the substrate, or to hinder further material growth, or the like.
Further preferred features include control systems 239 for automatically carrying out the methods of this invention with little or no operator intervention. Accordingly, the control systems receive sensor signals 241 preferably the progress of reactor chamber cleaning, and in dependence on the received sensor signals, provide control signals to robot arm controller, load lock door, etchant gas inlet in such a manner that the methods of this invention are carried out. Also the control systems can receive sensor signals monitoring the accumulation of undesired deposits. Control system 239 can also monitor and control other aspects of the operation of the reactor chamber and reactor chamber subsystems not specifically related to the methods of this invention. For example, control system 239 can also monitor and control reactor temperature, reactor pressure, precursor flow rates, and other aspects of the growth process. Control system 239 generally includes memory, storage, programmable devices, e.g., microprocessors, and the like. The control system also preferably includes user interface facilities, e.g., keyboard, display, and so forth.
The cleaning methods of this invention can be incorporated and integrated into this known process as follows. The controller continuously or intermittently senses the level of undesired deposits 309 in the reactor chamber, and when it determines that excessive undesired deposits 311 have accumulated, it terminates precursor gas flow 312 and performs reactor cleaning 313 according to this invention as illustrated by
Reactor cleaning 313 is preferably also controlled by control system 239 (or another specialized control system).
Such thermal damage is usually caused by surface decomposition or thermal stresses. If the stresses become excessive, the substrate can distort, e.g., by bowing or otherwise. If the substrate comprises layers having different coefficients of thermal expansion (CTE), the layers can crack or flake. In preferred embodiments of the invention external heating can be supplied to the substrate upon removal from the reactor to prevent damaging thermal shock. Alternatively heating elements can be housed within the transfer (load) chamber itself, although modification to internal components of the chamber maybe required to prevent component damage due to excessive temperatures. Alternatively, such thermal damage is avoided by preferably limiting rates of temperature change in the reactor chamber and also by lowering the reactor chamber temperature to nearer the ambient temperature (in the load chamber) when the substrates are moved in and out of the chamber. Accordingly,
It should be understood that the times and temperatures illustrated in
One growth-cleaning cycle is now described.
The cleaning process itself starts 317 with the first group of steps, steps 319 and 321, when control system 239 allows the reactor chamber temperature to ramp down 319 to a removal temperature which is illustrated as abut 500° C. This 600° C. temperature decline 403 is illustrated to require about 15 min. Next, the control system removes the substrate 321 by generating control signals 243 that in turn: open load lock door 215; instruct robot arm controller 231a to extend robot arm 231 through the open load lock door into reactor chamber 211; cause Bernoulli wand 233 to pick up the working substrate from susceptor 217; instruct the robot arm controller to retract the robot arm back into load chamber 213; and to close the load lock door. The substrate now held by the Bernoulli wand can optionally be placed on substrate holder 245 inside the load chamber (or on substrate holder 251 outside of the load chamber). The substrate holder can optionally be configured to buffer the temperature change of a working substrate. See, e.g., U.S. Pat. No. 6,893,507, which is included here by reference in its entirety for all purposes. Removal/replacement of the working substrate is illustrated here to require 1-2 min at 500° C.
Next, the second group of steps, steps 323, 325, 327, 329, 331, and 333, perform the actual removal of the undesired deposits. Cleaning is carried out at a higher cleaning temperature, which is illustrated 409 as about 1100° C., and accordingly the control system ramps 323 up the reactor chamber temperature from the lower removal/replacement temperature to the higher cleaning temperature. This ramp up 407 is illustrated as requiring about 6 min. Etchant gases can now be flowed 325 (in preferred embodiments) through the reactor chamber at selected flow rates and pressures to react with the undesired deposits forming gaseous products. Alternatively, a number of short cleaning cycles can be repeated, each cycle including admitting an aliquot of etchant gas into the chamber, retaining the gas in the chamber for a period of time, and then exhausting the gas. As an additional alternative, a chemically reactive plasma could be generated within the reactor chamber, e.g. by the application of a radio frequency electromagnetic field to the etchant gases, thereby creating high energy ionic species.
As described, the control system automatically monitors cleaning progress by, preferably, sampling the exhaust gases 327 from the reactor to determine the level of products of the cleaning reaction. If this level indicates that the reactor chamber is sufficiently clean 329, e.g., by falling below a threshold (or down to a trace level), the control system terminates the flow of etchant gas. The cleaning period 409 is here illustrated to be about 15 min. The etchant gases are preferably purged 331 from the reactor chamber prior to proceeding on to further growth by, e.g., flowing a purging gas through the chamber. In the case of GaN, H2 is preferably the purging gas and the reactor chamber is heated when the H2 is in the chamber. Next, the control system allows the reactor chamber temperature to ramp down 333 to the removal/replacement temperature range 413. This ramp down is illustrated as requiring about 9 min.
The last group of steps, steps 335 and 337, prepares the reactor chamber for a further period of epitaxial growth (if needed). The control system replaces the substrate back into reactor chamber 335 by controlling the load lock door and the robot arm to move the working substrate from the load chamber 335 (or from substrate holder 251 exterior to the load chamber) and place it on the susceptor in the reactor chamber. Other than being performed in a reverse order, the details of the replacement step 335 are essentially the same as those of removal step 321 and are not further described. Substrate replacement 413 is illustrated as requiring about 1-2 min., but may require a longer time if it includes semi-automatic or manual steps. Next, the reactor chamber temperature is ramped up 337 back to growth temperature 417, again illustrated as requiring about 15 min.
It is apparent from
Since, as discussed, the rates of temperature change and the removal/replacement temperatures are largely determined by the ability of the working substrate to withstand thermal stress, use of more thermally-resistant substrates is advantageous and a preferred approach for increasing the efficiency of the cleaning steps of this invention. Generally, it is preferable that the substrates and materials grown thereon (more generally, working materials used in a growth processes) be adapted to repeated transfer between a higher-temperature reactor chamber and a lower-temperature load chamber with little of no damage (that is, any damage does not impair intended uses of the working materials).
The response of a substrate to thermal stresses depends in part on its coefficient of thermal expansion (CTE). In certain preferred embodiments of the present invention, working substrates are substantially planar layers of a base substrate material on which are grown one or more further layers. If the CTEs of the various layers are sufficiently different or are sufficiently high, then lower thermal stresses or slower rates of temperature change can lead to differential thermal expansion sufficient to cause substrate damage. Therefore, substrates and the materials grown thereon advantageously have CTEs that are sufficiently low or sufficiently matched (in the case of heterogeneous materials) so that they can withstand higher thermal stresses and higher rates of temperature change without damage. For example, such substrates can be removed or replaced in a reactor chamber with a temperature greater than about 600° C., or about 700° C., or up to about 850° C. and higher.
CTEs can be matched in several ways. In one approach, a material can be grown on a base substrate of the same or a closely related material. For example, GaN (and other Group III-nitrides) can be grown on a base substrate of GaN itself, or GaN can be grown on a base substrate layer of, e.g., AlN which has a crystal structure and a CTE closely matched to those of GaN. See, e.g., U.S. Pat. No. 5,909,036, which is included herein by reference in its entirety for all purposes. In another approach, a composite base substrate can be constructed of one or more materials having CTEs matched to the material to be grown thereon and one or more other materials having crystal structures matched to the growth material. These materials are arranged so that the surface of the composite substrate is matched to the growth material in both CTE and crystal structure. For example, in the case of GaN (or other Group III-nitrides), a composite substrate can comprise two or more layers with the CTEs of upper layers (or layer) being increasingly better matched to the CTE of GaN and a, perhaps thin, surface layer with a crystal structure matched to GaN. See, e.g., U.S. Pat. No. 6,867,067 and US 2004/0235268.
For a typical cleaning process the robot arm will remove the growth wafer from the reactor chamber to the load-lock in a time period of less than one minute. The reactor chamber is heated to a temperature of between 650° C.-1200° C. Hydrogen flows into the chamber in conjunction with HCl vapor for more efficient activation of the etching species. Preferred embodiment also utilizes a dual flow process for optimized removal of reactor deposits. Initially, a low flow regime (5-10 slm H2+HCl, ration H2:HCl from 1:2 to 1:5) is employed to allow for the etchant species to diffuse through the entirety of the reactor chamber, ensuring that the etchant species can contact all areas of the reactor. For the second flow regime, the total flow rate is increased (10-40 slm H2+HCl, ration H2:HCl from 2:1 to 10:1). The high flow rate regime allows for the etching of material further downstream from the heated susceptor; in addition the high flow rate physically removes large particulates from the chamber walls. The total time period for cleaning of the chamber is between 5-30 minutes as determined from the signature of etch products from the FTIR analyzer. The growth wafer can then be reloaded for further nitride deposition.
The preferred embodiments of the invention described above do not limit the scope of the invention, since these embodiments are illustrations of several preferred aspects of the invention. Any equivalent embodiments are intended to be within the scope of this invention. Indeed, various modifications of the invention in addition to those shown and described herein, such as alternate useful combinations of the elements described, will become apparent to those skilled in the art from the subsequent description. Such modifications are also intended to fall within the scope of the appended claims. In the following (and in the application as a whole), headings and legends are used for clarity and convenience only. A number of references are cited herein, the entire disclosures of which are incorporated herein, in their entirety, by reference for all purposes. Further, none of the cited references, regardless of how characterized above, is admitted as prior to the invention of the subject matter claimed herein.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US08/85707 | 12/5/2008 | WO | 00 | 12/2/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 61015498 | Dec 2007 | US |
