Plasma Spraying and Recrystallization of Thick Film Layer

Abstract

A linear process tool comprising at least two deposition modules each comprising one or more plasma spray guns operable to move in a direction approximately orthogonal to the direction of a substrate carrier is configured to deposit at least a first and second layer, in direct contact with each other, wherein a first layer is of first composition and the second layer is of second composition different than the first composition.

Description

PRIORITY

This application claims benefit of provisional applications 61/181,496, filed May 27, 2009 and 61/296,799, filed Jan. 20, 2010.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. Nos. 11/881,501, 11/782,201, 12/074,651, 12/720,153, 12/749,160, 61/181,496, 61/305,796, 61/235,610, 61/239,739, 61/263,282, 61/296,799, 61/300,804; all owned by the same assignee and all incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to plasma spraying of silicon and other materials. In particular, the invention relates to the combination of plasma spraying and recrystallization of deposited material to form devices with advantageous structures.

2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98

Plasma spraying has been suggested over many years for forming silicon semiconducting devices including silicon solar cells. Such efforts have not found ready commercialization, because of the low quality of the sprayed silicon. Interest in plasma sprayed semiconducting silicon has been rekindled recently in the hope of providing a low cost manufacturing method for silicon solar cells. Zehavi et al. have disclosed a distinctive plasma spray gun for plasma spraying high-quality silicon in U.S.2008/0220558.

Sprayed silicon to date has suffered from the nature of plasma spraying in which the silicon arrives at a substrate 10, illustrated in the cross-sectional view of FIG. 1, as liquid drops 12 intermittently striking the surface and immediately cooling to form separate lamellae 14 in the form of overlapping plates or platelets, as is well known. Voids 16 may form between the lamellae. If the spraying is done at some ambient pressure, voids 16, containing oxygen or an ambient gas, are trapped within the silicon layer being formed. The oxygen degrades the quality of the silicon layer and its ability to function as a semiconductor. Even if the silicon is sprayed in an argon ambient, argon bubbles exist and provide scattering centers in the silicon, degrading at least its carrier mobility. For solar cells, the carrier diffusion length should be longer than the absorption length of the incident radiation.

Voids 16 contribute to the pervasive porosity of plasma spray material. Processing conditions can be adjusted to reduce the porosity, but it is believed that the voids 16 are inherently formed in plasma spraying. It thus appears necessary to recrystallize plasma sprayed silicon, that is, to heat the sprayed layer to a sufficiently high temperature to fuse the silicon lamellae into larger, void-free volume of crystalline silicon even if only polycrystalline silicon is needed. However, recrystallization has not been completely effective and it needs to be adapted to the production environment required for large, low-cost solar cells. The instant invention discloses an enhanced recrystallization method to be combined with low-pressure plasma spraying so that voids 16 are eliminated or at least minimized and preferably are eliminated from the deposited material by the recrystallization step.

Equipment is available to plasma spray within a vacuum chamber. However, this equipment is not readily adaptable for plasma spraying silicon solar cells. Typically, the plasma gun is mounted on a multi-jointed robot arm. Such a robot requires a large vacuum chamber; the arm is ill suited for coating large panels with a uniformly thick silicon layer needed for commercial solar cells; conventional vacuum chambers are not suitable for large scale production.

de Souza, et al. teach a technique for Grain Reorientation Annealing in U.S.2010/0112792; in a preferred embodiment a silicon layer is heated to less than 1350° C. in order to recrystallize, or more precisely, reorient, in the same orientation as a substrate on which the layer has been deposited. In U.S.2010/0075060, Narwankar discloses a process tool with micro-plasma spray guns for carbon nanotube growth on silicon substrates. In U.S.2008/0057212 Dorier, et al., disclose a plasma spraying device for spraying materials onto a substrate. In U.S. 2003/0113481 Huang discloses a method for depositing a coating onto a solid substrate employing a plasma source for fabricating a solar cell. In U.S. Pat. No. 4,379,020 Glaeser et al., disclose recrystallizing amorphous films into large polycrystalline films. None of the prior art teaches or suggests the limitations and advantages of the disclosed invention.

BRIEF SUMMARY OF THE INVENTION

In one embodiment of the instant invention a linear process tool comprising at least two deposition modules each comprising one or more plasma spray guns operable to move in a direction approximately orthogonal to the direction of a substrate carrier is configured to deposit at least a first and second layer, in direct contact with each other, wherein a first layer is of first composition and the second layer is of second composition different than the first composition. The plasma spray guns are of a type described in U.S. Ser. No. 12/074,651 wherein the plasma spray gun comprises components exposed to the plasma stream comprising at least one constituent of the primary source of material exposed to the plasma stream; the primary source of material for the spray gun for the deposited layer is a powder produced by a jet mill as described in U.S. Ser. No. 11/782,201. The substrate carrier conveys a substrate from an entrance to the linear process tool to an exit at a predetermined speed wherein the speed varies between about zero and 20 cm/min.

In one embodiment of the instant invention a linear process tool comprising at least two deposition modules also comprises at least one zone melt recrystallization, ZMR, module wherein a source of radiation is optically engineered into a narrow, linear line of radiation extending across the width of a substrate placed on the substrate carrier such that deposited material on the substrate is irradiated in the linear line of radiation and heated at least to the melting point of the deposited material; as a portion of a substrate with a deposited material layer is moved underneath the narrow, linear line of radiation by the substrate carrier the deposited material layer portion exposed to the radiation meets or exceeds its melting point; as the portion of the substrate with a deposited material layer passes out from underneath the narrow, linear line of radiation by the substrate carrier the deposited material layer portion now outside the radiated zone cools below its melting point and recrystallizes into a preferred orientation based on its composition. In some embodiments the recrystallized, deposited material layer exhibits a minority carrier diffusion length greater than 40 microns and a grain size larger than the layer thickness; in some embodiments a minority carrier diffusion length is greater than 20 microns.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 shows schematically as deposited lamellae resulting from a plasma spray.

FIG. 2 shows schematically a linear process tool.

FIG. 3 shows schematically a 3D version of a linear process tool.

FIG. 4 shows schematically an optically engineered radiation source.

FIG. 5 shows schematically an ultrasonic generator assisting in ZMR.

FIG. 6 shows reflection and absorption versus wavelength for silicon.

FIG. 7 shows schematically an optical system for creating a linear line of radiation.

FIG. 8 shows schematically an optical system for “beam homogenization”.

DETAILED DESCRIPTION OF THE INVENTION

According to one aspect of the invention, as schematically illustrated in FIG. 2, plasma spraying of silicon is performed in a vacuum deposition chamber 20 inside of which one or more plasma spray guns 22 having spray nozzles located within the chamber 20 spray silicon onto substrates 24 transported under the guns 22 by conveyor 26 or a series of moving stages. In this embodiment, the source of silicon is silicon powder injected into the plasma flow of argon and a small quantity of hydrogen to increase the thermal conductivity and enthalpy of the plasma. Improving the thermal conductivity and enthalpy improves the transfer of heat to the silicon. Other gases, such as helium can also be used for the same purpose. Unprocessed substrates 24 are loaded into a deposition chamber 20 from an entry load lock 28 through an isolation valve 30, for example a slit valve. Similarly, processed substrates are removed from the deposition chamber 20 to an exit load lock 32 through another isolation valve 34.

Plasma spraying commences when the deposition chamber has been pumped down to a desired pressure, for example, less than 500 milli-Torr, preferably with an oxygen fraction less than a critical value such as 10 ppm. One or more unprocessed substrates 24 are loaded by means well known in integrated circuit processing into the entry load lock 28 through a vacuum loading door, not shown, while entry isolation valve 30 to the deposition chamber is closed.

After the unprocessed substrates 24 have been loaded, the loading door is closed and the entry load lock 28 is vacuum pumped to a pressure substantially equal to that of the deposition chamber 20. A cassette can be used to load and store multiple substrates in the load lock 28 so as to reduce loading time and minimize pump down time. The unprocessed substrates 24 are sequentially loaded through the opened entry isolation valve 30 into the deposition chamber 20 for plasma spraying of silicon onto the substrates 24. It is appreciated that plasma spraying entails the use of a carrier gas, which is typically argon; optionally other gases may be used, including helium, hydrogen, etc. However, the deposition chamber 20 is continuously pumped to keep the argon pressure low. On the completion of processing, the processed substrate 24 is transferred through the opened exit isolation valve 34 into the exit load lock 32, which previously has been pumped down to deposition pressures. Again, a cassette may be in the exit load lock 32 to store multiple processed substrates 24. When the cassette is full or it is desired to remove the processed substrates 24, the exit isolation valve 34 is closed, a vacuum door on the exit load lock 32 is opened, and the cassette or individual substrates are removed from the exit load lock 32 into the ambient.

The illustrated system is an in-line processing system of the type well known in integrated circuit and display panel processing. It is understood that the deposition chamber need not include a separate exit load lock but use a single load lock for loading and unloading. It is also understood that the in-line system could include additional processing chambers, perhaps isolated from the plasma spraying chamber. A more detailed embodiment of the invention illustrated in the orthographic view of FIG. 3 includes an in-line configuration of multiple chambers mounted on a table 40 for processing substrates 42, for example, of graphite. Substrates may be a variety of shapes and materials depending upon the application; exemplary substrates are silicon, metallurgical grade silicon, graphite, carbon, silicon carbide coated graphite, flexible graphite and others known to one knowledgeable in the art. In some embodiments a flexible substrate comprises carbon and is between about 0.5 mm to about 5 mm thick; optionally a flexible substrate may have a silicon carbide outer coating; optionally a flexible substrate may be configured into a long ribbon more than 100 mm wide and more than 1,000 mm long; optionally a flexible substrate may be more than 10 meters long; optionally, long enough for roll-to-roll processing.

Multiple substrates 42 are loaded into a cassette 43. Two separately operated entry load locks 44, 46 can accommodate the cassette 43. The loading doors of both load locks are shown open although in typical operation they are opened only one at a time. The cassette 43 can be loaded outside of the load lock 44, 46 and then placed in the opened one of the load locks 44, 46 or the cassette 43 can be loaded with unprocessed substrates 42 while remaining in the opened one of the load locks 44, 46. The replication of entry load locks 44, 46 allows one of them to be loaded while the other is at vacuum and transferring substrates to the next stage.

Each load lock 44, 46 contains a robot 48 or other substrate handling mechanism to sequentially transfer unprocessed substrates 42 from the cassette 43 through an opened, unillustrated, isolation valve to the next stage. The isolation valve of the other load lock 44, 46 being loaded with fresh substrates needs to be closed during the loading.

In one embodiment, the first processing stage includes a first plasma spraying chamber having an overhead plasma gun 52 partially exposed outside the chamber 50 including connections 54, 56 for the argon spraying gas and cooling water and a vacuum-gated hopper 58 for the silicon powder and required dopant to be sprayed but having a spraying nozzle inside the chamber 50 which is maintained at a desired low pressure in the range of less than 200 Torr including the argon and hydrogen used in plasma spraying. Substrate support and moving means are included within the chamber 50 to support the substrate 42 transferred to it by the robot 48 in one of the load locks 44, 46. For example, the means may be an X-Y stage 60 which can move the supported substrate 42 under the plasma gun 52 in two-orthogonal directions in the plane perpendicular to the spraying axis to deposit a generally uniformly thick layer of silicon on the supported substrate 42. Alternatively, the means particularly for a circular substrate 42 may be a rotatable pedestal and the plasma gun 52 may include a vacuum-sealed linear slide in the roof of the chamber 50 so that the gun nozzle can move its spray plume along a radius of the rotating pedestal.

The illustrated plasma spraying chamber 50 can have a volume considerably smaller than conventional chambers described in the prior art. Chamber 50 is also configured to process a continuous stream of substrates while pumped down and not exposed to contaminating ambient. Accordingly, it is both economical to build and efficient to operate. Optionally, chamber 50 may be configured to process a “continuous substrate” such as a flexible substrate in a long continuous sheet or roll. Prior art systems are found in U.S. Pat. No. 6,186,090, U.S. Pat. No. 6,367,411, U.S. Pat. No. 6,488,995, U.S. Pat. No. 7,629,206; all incorporated by reference herein in their entirety.

In this embodiment, the next stage is a first transfer chamber 64 preferably but not necessarily having isolation valves, unillustrated, on its entry and exit sides and having a robot disposed therein which is capable of removing a partially processed substrate from the first plasma spraying chamber 50 through the opened entry isolation valve and transferring it to a second plasma spraying chamber 66 similar through the opened exit isolation valve. The second plasma spraying chamber 66 is similar to the first one 50 and also includes a plasma gun 68 and ancillary equipment inside and outside of the chamber 66. The replication of plasma spraying chambers is useful to increase the throughput, to produce a graded doped layer, for example p-type, and to isolate the spraying of n-type and p-type silicon if both are being sprayed. To provide proper isolation between the two plasma spraying chambers 50, 66, the entry and exit isolation valves on the first transfer chamber 64 should be opened alternately; that is, both should not be opened at the same time. One vacuum pumping system 70 may pump both the entry load/locks 44, 46 and the first transfer chamber 64 with separate valving to each of the pumped chambers. Another vacuum pumping system 72 may pump the two plasma spraying chambers 50, 66.

The stage or module following is a second transfer station 76 including its own robot and entry and exit isolation valves. When the substrate 42 has completed spray deposition in the second plasma spraying chamber 66, the robot in the second transfer chamber 76 moves it from the second plasma spraying chamber 66 to a recrystallization chamber 78 through alternately opened entry and exit isolation valves.

A recrystallization chamber or module 78 may assume various forms. The illustrated embodiment is configured for zone melt recrystallization (ZMR) and includes, optionally, a conveyor belt 82 moving a series of the substrates 42 through an RF coil 84. The RF coil 84, for example supplied with 13.56 MHz electrical power, couples sufficient RF energy into a limited area of the passing substrates 42 that the deposited film, for example silicon, melts or at least becomes mobile such that the lamellae 14 of FIG. 1 merge and the voids 16 are functionally eliminated. In ZMR, the molten zone progresses from one lateral side of the substrate 42 to the other so as to orient or regularize the crystallography in a single direction, for instance [100]. Vacuum transfer of substrates between the plasma spraying chambers 50, 66 and the recrystallization chamber 78 prevents the low-pressure voids 16 in the generally porous plasma sprayed silicon from filling with higher pressure gas during transfer.

Recrystallized substrates 42 cool as the conveyor belt 82 moves them towards two exit load locks 90, 92, here both shown in their opened state. Separate, unillustrated, isolation valves selectively isolate each of the exit load locks 90, 92 from recrystallization chamber 78. The exit load locks 90, 92 are configured and operate similarly to the entry load locks 44, 46 and allow the removal of fully processed substrates 42 from processing chambers held at low pressure during processing of a large number of substrates. A third vacuum pumping system 94 pumps the recrystallization chamber 78 and a fourth vacuum pumping system 96 pumps the second transfer station 76 and the two exit load locks 90, 92.

Other types energy may be used for of recrystallizing. For example, as illustrated in the side cross-sectional view of FIG. 4, an incandescent light source 100 linearly extending parallel to a surface of the substrate 42 emits radiation upon a reflector 102 positioned in back of the light source 100. The reflector 102 extends parallel to the linear light source 100 out of the plane of the illustration and is shaped in the illustrated plane to focus reflected radiation 104 as a narrow linear beam 106 at the surface of the substrate of sufficient intensity to locally heat the surface of the substrate to a desired recrystallization temperature. As the substrate 42 is moved in the illustrated horizontal direction, linear beam 106 moves across the surface of substrate 42 to effect ZMR. Additional optics, as shown in FIG. 7, which may be refractive, diffractive, reflective, or a combination of any or all types, performing the same or complementary functions as the reflector 102 may be positioned between the light source 100 and the substrate 42. Linear laser diode array 705 is an exemplary radiation source; first beam shaping lens 710 collects radiation and focuses into fly's eye lens 715, transmitting to collimator 720; second beam shaping lens 725 transmits to focusing lens 730 which focuses radiation into a narrow line incident at ZMR plane 750; ZMR plane defines a substrate surface 42 and location of the “melt line”. FIG. 7, collectively, comprises exemplary “optical components” sufficient to transform a radiation source, optionally a linear laser diode array 705, into a line source incident on a surface of moving substrate 42 at ZMR plane 750. Linear laser diode array 705 comprises a stack of line arrays of diodes. Along the line of the line array, divergence is low and is referred to as the slow axis. Along the stack of diodes divergence is larger and this is referred to as the fast axis. First beam shaping lens 710 is a lens or group of lenses to reduce aberrations in the slow axis. Fly's eye lens 715 comprises an array of small imaging lenses whose purpose is to homogenize the beam. Each array of the stack is imaged onto the same area after the fly's eye lens. Collimator 720 is a lens whose purpose is to bring both the fast and slow axes into minimum divergence. Second beam shaping lens 725 is a lens or group of lenses to correct for the uniformity of power intensity and create a desired beam profile in the ZMR plane. Focusing lens 730 focuses the collimated beam onto the substrate 42 surface at plane 750.

FIG. 8 shows schematically an optical system for “beam homogenization” based on the disclosure of Tanaka in U.S. Pat. No. 6,393,042. The fly's eye lens 715 comprises an array of lenslets which reproduce the linear diode array 705 once each lenslet. The end result is that each entire line is reimaged over every other line to create a homogeneous image focused at ZMR plane 750 producing a “melt line” on the surface of substrate 42. A more detailed discussion is found on the Suss MicroOptics web site, www.suss-microoptics.com/downloads/SMO_catalog.pdf [May 26, 2010], incorporated by reference herein.

Laser modules emitting directed beams and associated optics to produce similar scanned linear beams are described in U.S. Pat. No. 6,987,240. Alternatively, plasma guns similar to the plasma spray guns used for spraying silicon may be used to locally heat a substrate to a required recrystallization temperatures.

Eliminating voids and resulting densification of plasma sprayed material, optionally silicon, can be promoted during a recrystallization process, as illustrated in the schematic side cross-sectional view of FIG. 5, by supporting substrate 42 on a stiff support 110 coupled to ultrasonic generator 112, optionally, a piezo-electric transducer driven to produce kHz or MHz vibrations. Ultrasonic waves are coupled through substrate 42 to the surface irradiated by RF or radiant energy from linear beam 106. The ultrasonic waves promote the diffusion of the gaseous content of the voids 16 out of the molten or nearly molten silicon to collapse the voids 16 and densify the silicon.

In some embodiments linear processing as illustrated in FIG. 3 is adapted to a roll-to-roll system including one or more serially arranged plasma spraying chambers, or modules and one or more recrystallization chambers or modules. A supply roll of substrate material in flexible ribbon form is loaded into a location corresponding to an entry load lock. The substrate material may be stainless steel ribbon, graphite foil, molybdenum foil, ceramic foil or other suitable material in flexible ribbon form. An end of the ribbon is threaded through the serially arranged chambers and attached to a take up roll at a position corresponding to an exit load lock. Once the substrate ribbon is loaded into the system, the system is closed and pumped down to processing pressure. In some embodiments no transfer chambers or robots are needed between the chambers. If vacuum isolation is required, the isolation valves can be replaced by a pair of rollers closely accommodating the ribbon. Substrate ribbon is advanced through a deposition system and serially processed through various modules for deposition and ZMR, exiting onto a take up roll. At the completion of processing, a substrate ribbon is removed from a system; additional processing steps may be necessary for an intended purpose such as a processed solar cell or ceramic membrane. Other take up means may be substituted for the take up roll.

In some embodiments a plasma spray gun as described in U.S. Ser. No. 12/074,651 and silicon powder as described in U.S. Ser. No. 11/782,201 are used; in some embodiments different plasma spray guns and different materials are used.

In some embodiments a method of forming solar cells, comprises the steps of pumping at least one plasma spraying chamber; sequentially plasma spraying at a processing pressure of between 0.1 to 200 Torr either a roll or a plurality of substrates with silicon to form at least one silicon layer thereon; and recrystallizing the at least one silicon layer in a chamber held at a processing pressure of less than 200 Torr; optionally the roll or the substrates are not exposed to a pressure of more than 200 Torr between the plasma spraying and the recrystallizing steps; optionally, subjecting the at least one silicon layer to ultrasonic vibration during the recrystallizing step; optionally, loading a plurality of the substrates into a load lock chamber isolated from a spraying chamber use d for the plasma spraying; pumping the load lock chamber to the processing pressure; and transferring the substrates from the load lock chamber to the spraying chamber which has already been pumped to the processing pressure.

In some embodiments a processing system, comprises an entry load lock chamber capable of being loaded with a plurality of substrates and is pumped to a pressure of less than 200 Torr; one or more plasma spraying chambers configured to spray silicon coupled to the load lock chamber through an isolation valve and being capable of being pumped to a pressure of less than 200 Torr; optionally, a recrystallization chamber coupled to the one or more plasma spraying chambers at a pressure of less than 200 Torr and capable of recrystallizing silicon layers formed on substrates received from the one or more plasma spraying chambers; optionally, further comprising an exit load lock chamber capable of receiving a plurality of the substrates from the recrystallization chamber through an isolation valve and of being pumped to a pressure of less than 200 Torr.

In some embodiments a processing system, comprises a vacuum chamber capable of being pumped to less than 200 Torr; a supply roll of a ribbon substrate disposed in the chamber; a take up roll disposed in the chamber for taking up the ribbon substrate; at least one plasma spraying stations configured to spray silicon onto the ribbon substrate disposed in the chamber between the supply and take up rolls for spraying silicon on the ribbon substrate; optionally a recrystallization station disposed in the chamber between the at least one plasma spraying station and the take up roll and configured to recrystallize silicon sprayed on the ribbon substrate.

Zone melt recrystallization (ZMR) has been discussed and implemented in many applications requiring the formation of a high quality, low fault, crystal lattice after a material has been produced with substandard crystalline properties. Examples of this application are thin film depositions in solar cell fabrication or flat panel display devices. In both these cases, if the deposition is amorphous, there is a need to recrystallize the surface to achieve the required electrical properties of the device.

In bulk materials, float zone technology is very similar in method to achieve a similar result in which a narrow region of a crystal is molten, and this molten zone is moved along the crystal (in practice, the crystal is pulled through the heater). By controlling the speed of the bulk material through the molten area, crystal defects can repair themselves, or, impurities can be removed from the bulk material by being “pushed” forward by the melt zone.

The basic requirement of ZMR is to generate enough localized heat in order to melt a portion of the deposited material and to continue melting fresh material entering the zone as material leaving the zone solidifies and recrystallizes according to the crystalline structure of the material behind the melt zone, which acts as a seed. Common methods, well documented in the literature, used in solar applications use either a high power halogen lamp focused on the surface undergoing ZMR or a carbon strip heating element, heated by passing a high current through the strip, relying on the resistance of the carbon to generate heat. Both of these applications are capable of ZMR, but require significant control, and are not easily implemented in a manufacturing environment. Most systems in use are custom made by the end user, and each method has specific shortcomings. The halogen lamp systems are relatively unstable and difficult to control due to the natural fluctuations of the lamp filament and their relatively short lifetime. Additionally halogen lamp and carbon strip heating elements require significant base heaters to raise the overall temperature of the devices being processed to around 1000-1200° C. at which point, the ZMR is able to effectively recrystallize a layer of a few microns thickness, typically between about 2 to 5 microns. In some embodiments with a laser or LED diode array system with the disclosed optics a ZMR module is able to effectively recrystallize a layer of from a few microns thickness to more than 40 microns, typically between about 1 to 50 micron deposited layer thickness.

Another common application is based on excimer lasers and is in use for thin film transistor (TFT) flat panel displays (FPDs). The deposition for TFT FPDs puts down a layer of amorphous silicon typically measured in nanometers as compared to an optimum layer thickness of approximately 30 microns in solar applications. In other words, the layers deposited in TFTs are less than one-tenth the thickness of layers deposited in solar applications. Excimer laser recrystallization, as performed for TFT applications result in crystal domains of approximately 0.1 micron. The crystal domains needed in solar applications in order to achieve the necessary electronic properties are on the order of tens of microns, a difference of more than two orders of magnitude. Excimer lasers are used in TFT applications because the energy is absorbed in the surface and does not propagate into the bulk of the material. For solar applications of ZMR the energy must propagate into the silicon layer. In other words ZMR implementation in solar applications is a volume process, significantly differentiating it from existing excimer laser based ZMR for TFT panels.

Some embodiments of the instant invention comprise a linear array of diode lasers working at about 805 nm wavelength. A laser of this type may be a Coherent 4000L diode laser. By working at 805 nm, we see close to 70% of the incident light is absorbed by silicon (at 600 microns thickness), the remaining 30% is reflected, as shown in FIG. 6.

In some embodiments a linear array of lasers is imaged across the length of the surface being processed, which is typically 156 mm×156 mm for standard pseudo square solar cells. This creates a narrow line, approximately 0.5 to 4 mm wide, along the length of the solar cell. This line melts the surface silicon which has been deposited on the substrate and capped by an oxide layer to prevent agglomeration of the melted silicon into balls. The line output of the laser array is then scanned across the surface of the wafer, either using a slowly rotating mirror, a slow galvo controlled mirror, a robotic arm moving the entire laser head, or a motion control system moving the wafer underneath the line.

By moving the beam relative to the surface at a rate of approximately 1 mm/sec the beam continues to melt all unmelted surface area entering the line scan, while the surface exiting the line scan solidifies and recrystallizes in alignment with the crystal lattice of the material behind the melt zone. Because the preferred recrystallization is to the [100] plane, no seed crystal is required in the disclosed implementation of ZMR.

In another embodiment, a focused spot of radiation is scanned linearly across the surface being processed to create a “line” of melted material in the deposited thick film. This “melt line” is generated by a rotating minor or a galvo controlled minor. An optical system is implemented to keep the radiation beam in focus at all points of the “melt line”. The energy of the beam is adjusted to result in a continuous melt of the surface layer in the area of the beam. As in the previous embodiment disclosed, a “melt line” is moved relative to the surface at a rate of approximately 1 mm/sec; the beam continues to melt all unmelted surface area entering the “melt line” scan, while the surface exiting the “melt line” scan solidifies and recrystallizes in alignment with the crystal lattice of the material behind the melt zone.

In some embodiments a “melt line” is generated by cylindrical optics in conjunction with a radiation source; another embodiment generates a “melt line” by using diffractive optics. Another embodiment uses a high temperature source, optionally a hot plate, so that the surface being processed is elevated to a temperature close to the melting point of the material undergoing ZMR. This has the advantage of reducing the power requirements of a laser performing the ZMR, and, in some instances, reducing the thermal stresses generated by high temperature gradients in substrates. Use of a hot plate or other heat source to supplement a radiation source may result in less material losses due to stress related breakage.

In some embodiments a method of zone melt recrystallization of a layer of material comprises the steps of: scanning the surface of the layer with a laser beam such that the laser beam creates a thin line approximately 0.5 to 3 mm wide of molten phase material, and the line then progresses slowly across the surface of the layer in such a fashion that the material entering into the heated zone is melted and the material leaving the heated zone solidifies according to the crystal structure of the solid material behind the zone; optionally a liquid zone is created by rapidly scanning a spot of light such that the surface illuminated by the spot is a line that is in a continuously liquid phase; optionally, a liquid zone is created by imaging a linear array of light onto the surface, such that the surface illuminated by the line is in a continuously liquid phase; optionally, a liquid zone is created by imaging a spot of light onto a line image on the surface such that the surface illuminated by the line is in a liquid phase; optionally, a line scan is generated by a rapidly rotating mirror and appropriate optical system to maintain uniformity of beam size and energy density; optionally, a line scan is generated by a vibrating, galvonometrically controlled minor and appropriate optical system to maintain uniformity of beam size and energy density; optionally, a melt line is imaged using cylindrical optics; optionally, a melt line is imaged using diffractive optics; optionally, a melt line is slowly scanned across the surface of a layer by using a slowly rotated minor and appropriate optical system to maintain uniformity of beam size and energy density; optionally, a melt line is slowly scanned across the surface of the layer by using a galvonometrically controlled minor and appropriate optical system to maintain uniformity of beam size and energy density; optionally, a melt line is slowly scanned across the surface of a layer by moving the beam with a robotic arm; optionally, a melt line is slowly scanned across the surface of a layer by slowly moving the layer under the line; optionally, material being recrystallized is silicon; optionally, material being recrystallized is chosen from a group consisting of silicon, silicon-germanium alloys, Group IV elements and/or alloys, Group IV oxides, nitrides, carbides and mixtures thereof, metal oxides, nitrides, carbides and mixtures thereof, transition metal oxides, nitrides, carbides and mixtures thereof, rare earth metal oxides, nitrides, carbides and mixtures thereof; optionally, a layer being recrystallized is a layer of a solar cell; optionally, an additional heat source is used to increase the base temperature of a substrate in order to reduce the physical and thermal stress on the layer material in order to prevent breakage of the layer during zone melt recrystallization or after zone melt recrystallization; optionally, a base heater reaches a temperature just below the melting point of the layer being recrystallized; optionally, a system is enclosed in an environmentally controlled chamber to prevent chemical changes in the materials being processed due to interactions with a natural environment at elevated temperatures.

In some embodiments a linear process tool comprises first and second deposition modules each comprising one or more plasma spray guns operable to move in a direction approximately orthogonal to the direction of motion of a substrate carrier; wherein the first deposition module is configured to deposit a first layer on a substrate on the substrate carrier and the second deposition module is configured to deposit a second layer in direct contact with the first layer on the substrate on the substrate carrier such that the first layer is of first composition and the second layer is of second composition different than the first composition; wherein the plasma spray guns comprise components exposed to the plasma stream comprising at least one constituent of the primary source material in the plasma stream; wherein the primary source of materials for the plasma spray guns for the deposited layers is a first and second powder of first and second compositions produced by jet mills wherein the substrate carrier conveys the substrate from an entrance of the linear process tool to an exit at a predetermined speed; optionally a linear process tool further comprises a zone melt recrystallization module comprising a source of radiation; and optical components operable to convert the source of radiation into a linear line of radiation ranging from about 0.5 mm to 3 mm wide extending across the width of the substrate placed on the substrate carrier such that a portion of the deposited material layer on the substrate is irradiated in the linear line of radiation and heated at least to the melting point of the deposited material for a predetermined time; such that as the portion of the substrate with a deposited material layer passes out from underneath the linear line of radiation the deposited material layer portion now outside the radiated zone cools below its melting point and recrystallizes into a preferred orientation based on its composition; optionally, the recrystallized deposited material layer exhibits a minority carrier diffusion length greater than 40 microns and a grain size larger than the deposited material layer thickness; optionally, a linear process tool further comprises a means for ultrasonic vibration applied to the portion of the deposited material layer on the substrate being irradiated in the linear line of radiation and heated at least to the melting point; optionally, a substrate processed by the linear process tool is flexible; optionally, a the flexible substrate is configured such that it enters the linear process tool from a roll and exits the linear process tool on to a roll; optionally, the first and second compositions are chosen substantially from a group consisting of silicon, silicon-germanium alloys, Group IV elements and/or alloys, Group IV oxides, nitrides, carbides and mixtures thereof, metal oxides, nitrides, carbides and mixtures thereof.

In some embodiments a method for making a structure comprising first layer of first composition and second layer of second composition in contact wherein the first composition is different from the second composition comprising the steps:

depositing the first layer on a transported substrate with a first plasma spray gun operable to move in a direction approximately orthogonal to the transported direction; and
depositing the second layer on the transported substrate with a second plasma spray gun operable to move in a direction approximately orthogonal to the transported direction; wherein the first plasma spray gun comprises components exposed to the plasma stream comprising at least one constituent of a first primary source material in the plasma stream; wherein the first primary source material for the first plasma spray gun for the first layer is a first powder of the first composition produced by jet mills; wherein the second plasma spray gun comprises components exposed to the plasma stream comprising at least one constituent of a second primary source material in the plasma stream; wherein the second primary source material for the second plasma spray gun for the second layer is a second powder of the second composition produced by jet mills; optionally, the method for making a structure of claim 8 further comprises the step of heating a portion of the transported substrate to initiate zone melt recrystallization in a linear line ranging from about 0.5 mm to 3 mm wide extending across the width of the transported substrate to the melting point of the deposited material for a predetermined time; and cooling and recrystallizing the previously heated portion of the transported substrate into a preferred orientation based on its composition; optionally, a recrystallized portion of the transported substrate exhibits a minority carrier diffusion length greater than 40 microns and a grain size larger than the deposited material layer thickness; optionally, a method for making a structure further comprises the step:
applying ultrasonic vibration to the portion of the transported substrate being heated to initiate zone melt recrystallization; optionally, a transported substrate is flexible; optionally, the first and second compositions are chosen substantially from a group consisting of silicon, silicon-germanium alloys, Group IV elements and/or alloys, Group IV oxides, nitrides, carbides and mixtures thereof, metal oxides, nitrides, carbides and mixtures thereof.

The foregoing described embodiments of the invention are provided as illustrations and descriptions. They are not intended to limit the invention to a precise form as described. In particular, it is contemplated that functional implementation of invention described herein may be implemented equivalently in various combinations or other functional components or building blocks. Other variations and embodiments are possible in light of above teachings to one knowledgeable in the art of semiconductors, thin film deposition techniques, and materials; it is thus intended that the scope of invention not be limited by this Detailed Description, but rather by Claims following. All patents, patent applications, and other documents referenced herein are incorporated by reference herein in their entirety for all purposes.

In the preceding description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps and techniques, in order to provide a thorough understanding of the present invention. However, it will be appreciated by one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the invention.

It will be understood that when an element as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

Claims

1. A linear process tool comprising;

first and second deposition modules each comprising one or more plasma spray guns operable to move in a direction approximately orthogonal to the direction of motion of a substrate carrier; wherein the first deposition module is configured to deposit a first layer on a substrate on the substrate carrier and the second deposition module is configured to deposit a second layer in direct contact with the first layer on the substrate on the substrate carrier such that the first layer is of first composition and the second layer is of second composition different than the first composition; wherein the plasma spray guns comprise components exposed to the plasma stream comprising at least one constituent of the primary source material in the plasma stream; wherein the primary source of materials for the plasma spray guns for the deposited layers is a first and second powder of first and second compositions produced by jet mills wherein the substrate carrier conveys the substrate from an entrance of the linear process tool to an exit at a predetermined speed.

2. The linear process tool of claim 1 further comprising a zone melt recrystallization module comprising a source of radiation; and

optical components operable to convert the source of radiation into a linear line of radiation ranging from about 0.5 mm to 3 mm wide extending across the width of the substrate placed on the substrate carrier such that a portion of the deposited material layer on the substrate is irradiated in the linear line of radiation and heated at least to the melting point of the deposited material for a predetermined time; such that as the portion of the substrate with a deposited material layer passes out from underneath the linear line of radiation the deposited material layer portion now outside the radiated zone cools below its melting point and recrystallizes into a preferred orientation based on its composition.

3. The linear process tool of claim 1 wherein the recrystallized deposited material layer exhibits a minority carrier diffusion length greater than 40 microns and a grain size larger than the deposited material layer thickness.

4. The linear process tool of claim 1 further comprising means for ultrasonic vibration applied to the portion of the deposited material layer on the substrate being irradiated in the linear line of radiation and heated at least to the melting point.

5. The linear process tool of claim 1 wherein the substrate is flexible.

6. The linear process tool of claim 5 wherein the flexible substrate is configured such that it enters the linear process tool from a roll and exits the linear process tool on to a roll.

7. The linear process tool of claim 1 wherein the first and second compositions are chosen substantially from a group consisting of silicon, silicon-germanium alloys, Group IV elements and/or alloys, Group IV oxides, nitrides, carbides and mixtures thereof, metal oxides, nitrides, carbides and mixtures thereof.

8. A method for making a structure comprising first layer of first composition and second layer of second composition in contact wherein the first composition is different from the second composition comprising the steps:

depositing the first layer on a transported substrate with a first plasma spray gun operable to move in a direction approximately orthogonal to the transported direction; and

depositing the second layer on the transported substrate with a second plasma spray gun operable to move in a direction approximately orthogonal to the transported direction; wherein the first plasma spray gun comprises components exposed to the plasma stream comprising at least one constituent of a first primary source material in the plasma stream; wherein the first primary source material for the first plasma spray gun for the first layer is a first powder of the first composition produced by jet mills; wherein the second plasma spray gun comprises components exposed to the plasma stream comprising at least one constituent of a second primary source material in the plasma stream; wherein the second primary source material for the second plasma spray gun for the second layer is a second powder of the second composition produced by jet mills.

9. The method for making a structure of claim 8 further comprising the step of:

heating a portion of the transported substrate to initiate zone melt recrystallization in a linear line ranging from about 0.5 mm to 3 mm wide extending across the width of the transported substrate to the melting point of the deposited material for a predetermined time; and cooling and recrystallizing the previously heated portion of the transported substrate into a preferred orientation based on its composition.

10. The method for making a structure of claim 9 wherein the recrystallized portion of the transported substrate exhibits a minority carrier diffusion length greater than 40 microns and a grain size larger than the deposited material layer thickness.

11. The method for making a structure of claim 9 further comprising the step:

applying ultrasonic vibration to the portion of the transported substrate being heated to initiate zone melt recrystallization.

12. The method for making a structure of claim 9 wherein the transported substrate is flexible.

13. The method for making a structure of claim 8 wherein the first and second compositions are chosen substantially from a group consisting of silicon, silicon-germanium alloys, Group IV elements and/or alloys, Group IV oxides, nitrides, carbides and mixtures thereof, metal oxides, nitrides, carbides and mixtures thereof.