Patent Application
20070256635
The history of light-emitting diodes (“LEDs”) is sometimes characterized as a “crawl up the spectrum.” This is because the first commercial LEDs produced light in the infrared portion of the spectrum, followed by the development of red LEDs that used GaAsP on a GaAs substrate. This was, in turn, followed by the use of GaP LEDs with improved efficiency that permitted the production of both brighter red LEDs and orange LEDs. Refinements in the use of GaP then permitted the development of green LEDs, with dual GaP chips (one in red and one in green) permitting the generation of yellow light. Further improvements in efficiency in this portion of the spectrum were later enabled through the use of GaAlAsP and InGaAlP materials.
This evolution towards the production of LEDs that provide light at progressively shorter wavelengths has generally been desirable not only for its ability to provide broad spectral coverage but because diode production of short-wavelength light may improve the information storage capacity of optical devices like CD-ROMs. The production of LEDs in the blue, violet, and ultraviolet portions of the spectrum was largely enabled by the development of nitride-based LEDs, particularly through the use of GaN. While some modestly successful efforts had previously been made in the production of blue LEDs using SiC materials, such devices suffered from poor luminescence as a consequence of the fact that their electronic structure has an indirect bandgap.
While the feasibility of using GaN to create photoluminescence in the blue region of the spectrum has been known for decades, there were numerous barriers that impeded their practical fabrication. These included the lack of a suitable substrate on which to grow the GaN structures, generally high thermal requirements for growing GaN that resulted in various thermal-convection problems, and a variety of difficulties in efficient p-doping such materials. The use of sapphire as a substrate was not completely satisfactory because it provides approximately a 15% lattice mismatch with the GaN. Progress has subsequently been made in addressing many aspects of these barriers. For example, the use of a buffer layer of AlN or GaN formed from a metalorganic vapor has been found effective in accommodating the lattice mismatch. Further refinements in the production of Ga-N-based structures has included the use of AlGaN materials to form heterojunctions with GaN and particularly the use of InGaN, which causes the creation of defects that act as quantum wells to emit light efficiently at short wavelengths. Indium-rich regions have a smaller bandgap than surrounding material, and may be distributed throughout the material to provide efficient emission centers.
While some improvements have thus been made in the manufacture of such compound nitride semiconductor devices, it is widely recognized that a number of deficiencies yet exist in current manufacturing processes. Moreover, the high utility of devices that generate light at such wavelengths has caused the production of such devices to be an area of intense interest and activity. In view of these considerations, there is a general need in the art for improved methods and systems for fabricating compound nitride semiconductor devices.
Embodiments of the invention provide systems for fabricating compound nitride semiconductor structures. Each of various embodiments include a housing defining a processing chamber, a substrate holder disposed within the processing chamber, an NH3 source, a group-III precursor source, and an ultraviolet source. In a first set of embodiments, the system also includes a CVD showerhead disposed over the substrate holder. The showerhead comprises a first plenum fluidicly coupled with the NH3 source. The first plenum has a plurality of first channels fluidicly coupled with an interior of the processing chamber. The first plenum is optically coupled with the ultraviolet light source at an ultraviolet wavelength to receive light transmitted by the ultraviolet light source within the first plenum. The CVD showerhead also comprises a second plenum fluidicly coupled with the group-III precursor source. The second plenum has a plurality of second channels fluidicly coupled with the interior of the processing chamber.
In some of these embodiments, the ultraviolet wavelength is an NH3 absorption wavelength, with the ultraviolet light source either comprising a substantially monotonic light source having a wavelength substantially equal to the ultraviolet wavelength or transmitting light over a wavelength band that includes the ultraviolet wavelength. The group-III precursor source may comprise a gallium source. In some instances, the group-III precursor source comprises a plurality of distinct group-III elements.
The first plenum may comprise a window transmissive at the ultraviolet wavelength, with the ultraviolet source disposed relative to the window to transmit light into the first plenum. Examples of materials that may be comprised by the window include quartz and sapphire.
Alternatively, the system may additionally comprise an optical conduit transmissive at the ultraviolet wavelength. The optical conduit traverses at least a portion of the first plenum and is optically coupled with the ultraviolet light source to couple light from the ultraviolet light source into the first plenum. Examples of materials that may be comprised by the optical conduit include quartz and sapphire. The optical conduit may have a roughened outer surface. In one embodiment, the optical conduit comprises a plurality of optical fibers.
In a second set of embodiments, the system also includes a CVD showerhead disposed over the substrate holder. The CVD showerhead comprises first and second plenums. The first plenum is fluidicly coupled with the NH3 source and has a plurality of first channels fluidicly coupled with an interior of the processing chamber. The second plenum is fluidicly coupled with the group-III precursor source and has a plurality of second channels fluidicly coupled with the interior of the processing chamber. In these embodiments, the system additionally comprises an optical conduit optically coupled with the ultraviolet source. The optical conduit traverses through the CVD showerhead to transmit light from the ultraviolet light source onto a surface of a substrate disposed over the substrate holder.
The optical conduit may sometimes comprise quartz or sapphire. In certain embodiments, the optical conduit comprises a plurality of optical fibers. Each optical fiber is optically coupled with the ultraviolet light source and traverses through the CVD showerhead to transmit light from the ultraviolet light source onto the surface of the substrate. In different embodiments, at least one of the optical fibers passes through one of the first channels or at least one of the optical fibers passes through one of the second channels.
In a third set of embodiments, the system includes a CVD showerhead disposed over the substrate holder. In these embodiments, the CVD showerhead comprises a first plurality of tubes and a second plurality of tubes. Each of the first plurality of tubes has substantially concentric inner and outer channels. The inner channel is fluidicly coupled with the group-III precursor source and the outer channel is adapted for carrying a coolant flow. Each of the second plurality of tubes is fluidicly coupled with the NH3 source and is optically coupled with the ultraviolet source.
In one embodiment, the coolant flow comprises a water flow. An interior surface of at least one of the second plurality of tubes may be roughened and/or may comprise an ultraviolet coating. In one embodiment, at least one of the second plurality of tubes comprises an optical lightguide optically coupled with the ultraviolet light source and disposed within an interior of the at least one of the second plurality of tubes.
In a fourth set of embodiments, the system comprises a crossflow channel. The crossflow comprises a material transmissive to an ultraviolet wavelength and comprises a separation plate to define a plurality of subchannels. A first of the subchannels is fluidicly coupled with the group-III precursor source and is configured to provide a flow of a group-III precursor over a substrate disposed over the substrate holder in a direction substantially parallel to a surface of the substrate. A second of the subchannels is fluidicly coupled with the NH3 source and configured to provide a flow of NH3 over the substrate in a direction substantially parallel to the surface of the substrate. The ultraviolet light source is in optical communication with the crossflow channel and configured to transmit light at the ultraviolet wavelength through the material to activate the NH3.
In some of these embodiments, the ultraviolet light source is configured to transmit light through the material at a position along the crossflow channel before termination of the separation plate into the second of the subchannels. In others of these embodiments, the ultraviolet light source is configured to transmit light through the material at a position along the crossflow channel after termination of the separation plate where the group-III precursor and the NH3 mix. For instance, the position may be over the substrate holder. The separation plate may comprise an ultraviolet reflective coating on a side defining the second of the subchannels.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sublabel is associated with a reference numeral and follows a hyphen to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sublabel, it is intended to refer to all such multiple similar components.
1. Overview
One class of techniques for deposition of group-III nitride structures is metalorganic chemical vapor deposition (“MOCVD”). Such techniques achieve deposition by providing flows of precursors for both the group-III element(s) and nitrogen to a processing chamber where thermal processes act to achieve growth of a III-N film. The effectiveness of the growth may depend on a wide array of different factors, notably including the rate at which precursors are flowed into the processing chamber and the environmental conditions within the processing chamber. Particularly relevant to effective deposition of III-N films is the temperature within the processing chamber, although other conditions such as the pressure are also relevant.
The most common nitrogen precursor used in such processes is NH3, which is a very stable molecule that is difficult to pyrolyze thermally. The deposition process requires that the NH3 dissociate to make activated nitrogen available for reaction in growing the film. The difficulty of pyrolyzing NH3 is accommodated in a number of ways that generally reflect otherwise undesirable processing conditions. For example, because dissociation rates are relatively low, very large flows of NH3 are typically needed to provide sufficient activated nitrogen for reaction, resulting in large consumption rates. In addition, processing temperatures are also somewhat elevated in part to provide an environment that increases the dissociation rate. Deposition of III-N films is typically performed at temperatures that exceed 1000° C. But the use of very high temperatures exacerbates difficulties that result from differences in thermal expansion between the nitride film and the substrate over which it is deposited. These differences in thermal expansion are manifested by a distortion in the shape of the structure when the substrate is cooled, either as part of a multistep process or at completion of a process for fabricating a specific structure. Such distortions may result in wavelength and light-output variations for devices formed across the structure.
In addition, the inventors have recognized that relatively low dissociation rate of NH3 has a generally detrimental effect on the optoelectronic properties of the III-N film that is deposited. This may be understood in the context of the chemistry that governs defect formation in nitride structures. The inventors believe that nitrogen vacancies in III-N films are nonradiative, a hypothesis that results from analogy with vacancies in other III-V semiconductor structures in which the group-V vacancies are known to be nonradiative. The difficulty of pyrolyzing NH3 results in relative fewer nitrogen vacancies being formed in the deposited film, which in turn causes the overall internal quantum efficiency of the deposited film to be around 40-50%. The inventors anticipate that a mechanism for increasing the overall dissociation rate of NH3 will reduce the number of nitrogen vacancies formed by providing more activated nitrogen for reaction and thereby increase the internal quantum efficiency, perhaps to about 80-90%. This hypothesis is consistent with observations that improved optoelectronic properties are obtained in processes performed at higher NH3 partial pressures.
An increase in NH3 dissociation is achieved in embodiments of the invention by using ultraviolet light for activation, in addition to pyrolysis mechanisms. The improved activation permits a reduction in flow rates of NH3 that results in corresponding reduction in overall NH3 consumption, permits a reduction in processing temperature that mitigates the shape distortions that arise from thermal-expansion differences, permits an increase in overall film growth rates, and/or permits the fabrication of films having improved optoelectronic properties. Different embodiments of the invention may use substantially monochromatic light that match the NH3 absorption wavelength or may use a band of light that includes the NH3 absorption wavelength. Use of substantially monochromatic light is generally more efficient.
One typical nitride-based structure is illustrated in
2. Exemplary Substrate Processing System
The CVD apparatus includes an enclosure assembly 237 that forms vacuum chamber 215 with a gas reaction area 216. A gas distribution structure 221 disperses reactive gases and other gases, such as purge gases, toward one or more substrates 209 held in position by a substrate support structure 208. Between gas distribution structure 221 and the substrate 209 is gas reaction area 216. Heaters 226 can be controllably moved between different positions to accommodate different deposition processes as well as for an etch or cleaning process. A center board (not shown) includes sensors for providing information on the position of the substrate.
Different structures may be used for heaters 226. For instance, some embodiments of the invention advantageously use a pair of plates in close proximity and disposed on opposite sides of the substrate support structure 208 to provide separate heating sources for the opposite sides of one or more substrates 209. Merely by way of example, the plates may comprise graphite or SiC in certain specific embodiments. In another instance, the heaters 226 include an electrically resistive heating element (not shown) enclosed in a ceramic. The ceramic protects the heating element from potentially corrosive chamber environments and allows the heater to attain temperatures up to about 1200° C. In an exemplary embodiment, all surfaces of heaters 226 exposed to vacuum chamber 215 are made of a ceramic material, such as aluminum oxide (Al2O3 or alumina) or aluminum nitride. In another embodiment, the heaters 226 comprises lamp heaters. Alternatively, a bare metal filament heating element, constructed of a refractory metal such as tungsten, rhenium, iridium, thorium, or their alloys, may be used to heat the substrate. Such lamp heater arrangements are able to achieve temperatures greater than 1200° C., which may be useful for certain specific applications.
Reactive and carrier gases are supplied from gas delivery system 220 through supply lines to the gas distribution structure 221. In some instances, the supply lines may deliver gases into a gas mixing box to mix the gases before delivery to the gas distribution structure. In other instances, the supply lines may deliver gases to the gas distribution structure separately, such as in certain showerhead configurations described below. Gas delivery system 220 includes a variety of gas sources and appropriate supply lines to deliver a selected amount of each source to chamber 215 as would be understood by a person of skill in the art. Generally, supply lines for each of the gases include shut-off valves that can be used to automatically or manually shut-off the flow of the gas into its associated line, and mass flow controllers or other types of controllers that measure the flow of gas or liquid through the supply lines. Depending on the process run by the system, some of the sources may actually be liquid sources rather than gases. When liquid sources are used, gas delivery system includes a liquid injection system or other appropriate mechanism (e.g., a bubbler) to vaporize the liquid. Vapor from the liquids is then usually mixed with a carrier gas as would be understood by a person of skill in the art. During deposition processing, gas supplied to the gas distribution structure 221 is vented toward the substrate surface (as indicated by arrows 223), where it may be uniformly distributed radially across the substrate surface in a laminar flow.
Purging gas may be delivered into the vacuum chamber 215 from gas distribution structure 221 and/or from inlet ports or tubes (not shown) through the bottom wall of enclosure assembly 237. Purge gas introduced from the bottom of chamber 215 flows upward from the inlet port past the heater 226 and to an annular pumping channel 240. Vacuum system 225 which includes a vacuum pump (not shown), exhausts the gas (as indicated by arrows 224) through an exhaust line 260. The rate at which exhaust gases and entrained particles are drawn from the annular pumping channel 240 through the exhaust line 260 is controlled by a throttle valve system 263.
The temperature of the walls of deposition chamber 215 and surrounding structures, such as the exhaust passageway, may be further controlled by circulating a heat-exchange liquid through channels (not shown) in the walls of the chamber. The heat-exchange liquid can be used to heat or cool the chamber walls depending on the desired effect. For example, hot liquid may help maintain an even thermal gradient during a thermal deposition process, whereas a cool liquid may be used to remove heat from the system during other processes, or to limit formation of deposition products on the walls of the chamber. Gas distribution manifold 221 also has heat exchanging passages (not shown). Typical heat-exchange fluids water-based ethylene glycol mixtures, oil-based thermal transfer fluids, or similar fluids. This heating, referred to as heating by the “heat exchanger”, beneficially reduces or eliminates condensation of undesirable reactant products and improves the elimination of volatile products of the process gases and other contaminants that might contaminate the process if they were to condense on the walls of cool vacuum passages and migrate back into the processing chamber during periods of no gas flow.
The system controller controls activities and operating parameters of the deposition system. The system controller may include a computer processor and a computer-readable memory coupled to the processor. The processor executes system control software, such as a computer program stored in memory. The processor operates according to system control software (program), which includes computer instructions that dictate the timing, mixture of gases, chamber pressure, chamber temperature, microwave power levels, pedestal position, and other parameters of a particular process. Control of these and other parameters is effected over control lines that communicatively couple the system controller to the heater, throttle valve, and the various valves and mass flow controllers associated with gas delivery system 220.
3. Exemplary Embodiments
An overview of embodiments of the invention is illustrated with the flow diagram of
In some instances, additional group-V precursors may also be included in the flows at block 316. For example, a III-N-P structure may be fabricated by including a flow of phosphine PH3 or a III-N-As structure may be fabricated by including a flow of arsine AsH3. The relative stoichiometry of the nitrogen to the other group-V element in the deposited structure may be determined by suitable choices of relative flow rates of the respective precursors. In still other instances, doped compound nitride structures may be formed by including dopant precursors, particular examples of which include the use of rare-earth dopants.
The processing conditions established at blocks 312 and the precursor flows provided at block 316 may vary depending on specific applications. The following table provides exemplary processing conditions and precursor flow rates that are generally suitable in the growth of nitride semiconductor structures using the devices described above:
As will be evident from the preceding description, a process might not use flows of all the precursors identified in the table in any given process.
Dissociation of the NH3 is promoted through application of ultraviolet light at block 320. As noted above, such application permits the overall flow rate of NH3 to be provided in the lower portion of the cited range so that in some embodiments, the NH3 flow rate is less than 10,000 sccm, is less than 5000 sccm, is less than 2000 sccm, is less than 1000 sccm, is less than 500 sccm, or is less than 200 sccm. The application of ultraviolet light also permits more effective use of temperatures in the lower portion of the cited fange, with the temperature being less that 1000° C. in some embodiments. The overall growth rates when using such a process with application of ultraviolet light may exceed 3 μm/h, may exceed 4 μm/h, may exceed 5 μm/h, may exceed 6 μm/h, may exceed 7 μm/h, or may exceed 8 μm/h. In some embodiments, the ultraviolet activation is performed upstream of the substrate, while in other embodiments, a surface of the substrate is illuminated in performing the ultraviolet activation. Examples of structures that permit each of these types of activation are provided below.
Deposition of the III-N film at block 324 is thus achieved through the combination of providing suitable precursor flows under suitable processing conditions with ultraviolet activation of NH3. Once the film has been deposited, the precursor flows are terminated at block 328. In some instances, this may be followed by further processing on the substrate, such as by depositing additional nitride or other layers. Such additional processing steps may be performed within the same processing chamber or the substrate may be transferred to a second processing chamber for further processing. When processing is performed in multiple chambers, further increases in productivity may result from performing specific processes in chambers adapted for efficient performance of those processes. Further description of a cluster tool that includes multiple chambers that may be used for such multichamber processes is described in copending, commonly assigned U.S. patent application Ser. No. ______, entitled “EPITAXIAL GROWTH OF COMPOUND NITRIDE SEMICONDUCTOR STRUCTURES,” filed by Sandeep Nijhawan et al. (Attorney Docket No. A 10938/T68100), the entire disclosure of which is incorporated herein by reference for all purposes.
One exemplary structure that may be used to provide ultraviolet activation of NH3 is shown in
A window 432 that is at least partially transparent to ultraviolet light is provided to the plenum through which the NH3 is provided. Ultraviolet light 436 is generated by an ultraviolet source 440 and transmitted through the window to activate the NH3. The NH3 is thus excited into an active state before injection into the processing chamber 404. Examples of suitable window materials include sapphire and quartz, although other materials that are at least partially transmissive at ultraviolet wavelengths may alternatively be used. The ultraviolet source 440 may be substantially monochromatic at the activation wavelength of NH3 or may provide a wavelength band that includes the activation wavelength.
The optical fiber(s) 532 are made of a material transmissive at ultraviolet wavelengths, such as sapphire. In certain embodiments, the outer surface of the optical fiber(s) is roughened to improve scattering of the ultraviolet light out of the fiber(s) and into the second plenum. Different mechanisms for propagating light through the optical fiber(s) may be used. For instance, in many embodiments, propagation is unidirectional, with the ultraviolet source 536 being provided at one end of the fiber(s). In other instances, propagation may be bidirectional through the fiber(s) with a light source being provided at each end of the fiber(s). One such arrangement is illustrated in
Because light is provided to the second plenum through fibers passing through the plenum, there is greater flexibility in which plenum is to be used for particular precursors. In particular, while the illustration of
Another arrangement that uses optical fibers with a showerhead is illustrated in
Illumination with ultraviolet light incident on the substrate surface generally provides greater NH3 activity than upstream excitation as achieved with the arrangements of
Illustrations of configurations that may be used for ultraviolet activation with a crossflow reactor are shown in
The crossflow channel 704 is fabricated of a material transmissive at ultraviolet wavelengths, such as sapphire or quartz, permitting illumination with ultraviolet light 728 or 728′through the channel 704 by an ultraviolet source (not shown). Illumination may take place at a positions along the channel 704 before termination of the separation plate 708 on the side of the NH3 subchannel as indicated in
In arrangements where the illumination takes place before termination of the separation plate 708, it may be advantageous for the surface of the separation plate to be coated with an ultraviolet reflector, at least on the subchannel side to be used for NH3 flow. This permits more efficient activation by providing a double-pass for the ultraviolet light. Suitable coating materials include refractory metals like molybdenum, niobium, tantalum, rhenium, or tungsten, or may include a dielectric stack reflector.
Excitation at the substrate surface similar to that described in connection with
An alternative showerhead arrangement that may be used in embodiments of the invention is illustrated in
A structure that may be used for each of the second tubes 816 is shown in cross section in
Examples of structures that may be used for the first tubes 812 are shown in cross section in
Having fully described several embodiments of the present invention, many other equivalent or alternative methods of producing the cladding layers of the present invention will be apparent to those of skill in the art. These alternatives and equivalents are intended to be included within the scope of the invention, as defined by the following claims.
