WO2011127619A1

WO2011127619A1 - A method and apparatus for depositing a microcrystalline material in photovoltaic applications

Info

Publication number: WO2011127619A1
Application number: PCT/CH2011/000080
Authority: WO
Inventors: Markus Klindworth; Markus Kupich
Original assignee: Oerlikon Solar Ag, Trübbach
Priority date: 2010-04-16
Filing date: 2011-04-14
Publication date: 2011-10-20
Also published as: EP2558612A1; CN102834546A; CN105887040A; JP2013529374A; KR20130093490A; CN102834546B

Abstract

A deposition method and system for producing a photovoltaic cell is provided. The method includes maintaining a sub-atmospheric pressure within a reaction chamber during at least a portion of the deposition of the semiconductor material. The distance D separating the first and second electrodes is expressed in mm, and is greater than or equal to about 10 mm but less than or equal to about 30 mm. A concentration of the semiconductor-containing gas in the process gas of at least fifty (50%) percent by volume is established during at least a portion of the deposition of the semiconductor material.

Description

A METHOD AND APPARATUS FOR DEPOSITING A MICROCRYSTALLINE MATERIAL IN PHOTOVOLTAIC APPLICATIONS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 61/324,909, filed April 16, 2010, which is incorporated in its entirety herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0002] This application relates generally to a method and apparatus for creating solar cells and, more specifically to method and apparatus for depositing microcrystalline silicon layers on a substrate for thin-film solar cells.

2. Description of Related Art

[0003] Photovoltaic devices, also referred to as photoelectric conversion devices or solar cells, are devices that convert light, especially sunlight into direct current (DC) electrical power. For low-cost mass production thin-film solar cells are of particular interest since they allow glass, glass ceramics or other rigid or flexible materials to be used as a substrate instead of crystalline or polycrystalline silicon. The solar cell structure, i.e., the layer sequence responsible for, or capable of, producing the photovoltaic effect is deposited in thin layers on the substrate. This deposition may take place under atmospheric or vacuum conditions. Deposition techniques are widely known in the art, such as PVD, CVD, PECVD, APCVD, etc ., each of which is used in the production of semiconductor devices.

[0004] A thin-film solar cell generally includes a first electrode, one or more semiconductor thin-film p-i-n junctions, and a second electrode, which are successively stacked on a substrate. Each p-i-n junction or thin-film photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n-type layer (p-type = positively doped, n-type = negatively doped). The i-type layer, which is a substantially intrinsic semiconductor layer, occupies a majority of the thickness of the thin-film p-i-n junction and is primarily responsible for the photoelectric conversion performed by the solar cell.

[0005] As thin-film solar cells are increasingly mass-produced, integrated manufacturing processes for efficiently and effectively manufacturing such solar cells are required. Conventional manufacturing processes such as plasma-enhanced chemical vapor deposition ("PECVD") have traditionally utilized a reactant that is highly diluted with hydrogen or other diluting gas. For instance, silane (SiH₄) gas has traditionally been diluted with H₂ to concentrations below 10% by volume, requiring large amounts of hydrogen (H₂), which ultimately exits the deposition chamber without reacting with another reactant. In other words, a significant portion of the large quantities of H₂ introduced into the deposition chamber is intended to only dilute the silane (S1H₄) gas. This significant portion of the hydrogen (H₂) diluting agent does not otherwise contribute to the formation of layers on the substrate of the solar cell, and is exhausted as a waste product requiring disposal. The total flow of gas during deposition is one of the primary factors influencing the size of pumps, tubes, gas supplies and amount of waste materials requiring disposal for such processes, contributing to expensive production costs.

[0006] Another function of a large hydrogen (H₂) volumetric flow rate in conventional processes such as that describe above is to flush out silicon (Si)-containing products formed in the plasma volume. However, the large hydrogen (H₂) volumetric flow rate also entrains and removes non-dissociated (or not-fully dissociated) silane (S1H₄) from the deposition chamber. This premature removal of silane (S1H₄) gas from the deposition chamber causes silane (SiH₄) gas to be inefficiently consumed and excess amounts of silane (S1H₄) gas (and partially-dissociated silane (S1H₄)) to be exhausted, requiring disposal. Both conditions add to the overall product cost of thin-film solar cells.

[0007] Further, high process pressures established within deposition chambers during conventional solar-cell fabrication processes decrease the mean free path for molecules in the process area. These elevated process pressures promote the growth of particulate silicon-containing products in the plasma instead of on the substrate, resulting in a low deposition rate that increases the overall time required for production of thin- film solar cells.

[0008] Dopant gases such as Phosphine (PH₃) and Trimethylboron (B(CH₃)₃) negatively influence the nucleation of microcrystalline silicon when preparing doped microcrystalline silicon layers according to conventional fabrication processes. To counteract such negative influence, P-type and n-type doped microcrystalline silicon layers therefore have traditionally been prepared at an increased hydrogen dilution and lower total volumetric silane (S1H₄) flows compared to the preparation of intrinsic microcrystalline silicon. But in addition to the concerns addressed above, the increased hydrogen dilutions result in a lower deposition rate. As a consequence, even though the thickness of doped layers of a solar cell (few tens of nm) is less than that of the intrinsic layer, the time consumed to deposit the doped layers has a significant impact on the overall time requirement to fabricate such solar cells.

[0009] Attempts to overcome the shortcomings noted above have typically involved completely different types of deposition, often involving the generation of plasmas using pure silane (S1H₄) (i.e., undiluted). However, these different deposition processes require significant modifications of existing commercially-available, large-area PECVD deposition machines designed for deposition processes utilizing diluted silane (S1H₄) due to differences in gas flow stability and silicon powder trapping.

BRIEF SUMMARY

]0010] According to one aspect, the subject application involves a deposition system for producing a photovoltaic cell, including a deposition chamber that substantially encloses a reaction space where a semiconductor material is to be deposited onto a substrate to form a microcrystalline layer of the semiconductor material on the substrate. A substrate conditioner provides a heating effect to the substrate, provides a cooling effect to the substrate, or provides heating and cooling effects to the substrate to establish a desired temperature of the substrate for deposition of the semiconductor material. First and second electrodes that oppose each other are separated by a distance D, and are operatively connected to a power source to be energized for igniting a plasma and maintaining the plasma in the reaction space during at least a portion of the deposition. A vacuum subsystem at least partially evacuates the deposition chamber, and a delivery subsystem introduces a process gas to the reaction space. The process gas includes a semiconductor-containing gas from a semiconductor source and a diluting agent from a dilution source. A controller is programmed to control operation of at least one of the vacuum subsystem and the delivery subsystem to maintain the sub- atmospheric pressure during at least a portion of the deposition of the semiconductor material at a pressure that is less than or equal to:

50 mbar · mm

D

The distance D separating the first and second electrodes is expressed in mm. The controller is also programmed to establish a concentration of the semiconductor- containing gas in the process gas of at least fifty (50%) percent by volume during at least a portion of the deposition of the semiconductor material.

[0011] According to another aspect, the subject application involves a method of depositing a semiconductor material onto a substrate in a deposition chamber of a deposition system. The deposition system also includes a substrate conditioner that provides one or both of a heating effect and a cooling effect to the substrate, first and second electrodes separated by a distance D and operatively connected to a power source for establishing a plasma in the deposition chamber, a vacuum subsystem for at least partially evacuating the deposition chamber, and a delivery subsystem for introducing a process gas to the deposition chamber. The method includes, with a controller, receiving a sub-atmospheric pressure to be established within the deposition chamber during at least a portion of the deposition of the semiconductor material. The sub-atmospheric pressure is less than or equal to:

50 mbar^■ mm

D

The distance D separating the first and second electrodes is expressed in mm. The method also includes transmitting a pressure signal that controls operation of the vacuum subsystem to at least partially evacuate the deposition chamber and establish the sub- atmospheric pressure received with the controller. Also with the controller, a target temperature of the substrate for the deposition is received. A temperature signal that controls the substrate conditioner to elevate, lower or both elevate and lower a temperature of the substrate to a temperature approaching or approximately equal to the target temperature is transmitted from the controller. A plasma signal that controls the power source to energize the first and second electrodes and establish the plasma within the deposition chamber is also transmitted from the controller. A flow signal that controls operation of a delivery subsystem is transmitted from the controller to introduce a semiconductor-containing gas and a suitable quantity of a diluting agent into the deposition chamber to establish a concentration of the semiconductor-containing gas within the deposition chamber of at least fifty (50%) percent by volume during at least a portion of the deposition.

[0012] According to another aspect, the subject application involves a method of depositing a semiconductor material onto a substrate in a deposition chamber of a deposition system. The deposition system also includes a substrate conditioner that provides one or both of a heating effect and a cooling effect to the substrate, and first and second electrodes separated by a distance D and operatively connected to a power source for establishing a plasma in the deposition chamber. A vacuum subsystem is provided to at least partially evacuate the deposition chamber, and a delivery subsystem introduces a process gas to the deposition chamber. The method includes establishing a sub- atmospheric pressure within the deposition chamber during at least a portion of the deposition of the semiconductor material, the sub-atmospheric pressure being less than or equal to:

50 mbar■ mm

D

The distance D separating the first and second electrodes is expressed in mm. Using the substrate conditioner, a temperature of the substrate is elevated, lowered or both elevated and lowered to a temperature approaching, or approximately equal to a target temperature within a range from about 120°C to about 280°C. Using the power source, at least one of the first and second electrodes is energized to establish the plasma within the deposition chamber. A semiconductor-containing gas and a suitable amount of the diluting agent are introduced to the deposition chamber to establish a concentration of the semiconductor- containing gas within the deposition chamber of at least fifty (50%) percent by volume during at least a portion of the deposition.

[0013] According to another aspect, the subject application involves a method of depositing a semiconductor material onto a substrate in a deposition chamber of a deposition system. The deposition system also includes a substrate conditioner that provides one or both of a heating effect and a cooling effect to the substrate, first and second electrodes separated by a distance D and operatively connected to a power source for establishing a plasma in the deposition chamber, a vacuum subsystem for at least partially evacuating the deposition chamber, and a delivery subsystem for introducing a process gas to the deposition chamber. The method includes establishing a sub- atmospheric pressure within the deposition chamber during at least a portion of the deposition of the semiconductor material, the sub-atmospheric pressure being less than or equal to:

50 mbar mm

~D

The distance D separating the first and second electrodes is expressed in mm and is greater than or equal to about 10 mm but less than or equal to about 30 mm. Using the substrate conditioner, a temperature of the substrate is elevated, lowered, or both elevated and lowered to a temperature approaching, or approximately equal to a target temperature within a range from about 120°C to about 280°C. Using the power source, at least one of the first and second electrodes is energized to establish the plasma within the deposition chamber. A semiconductor-containing gas and a suitable amount of the diluting agent are introduced to the deposition chamber for the deposition to be performed.

[0014] The above summary presents a simplified summary in order to provide a basic understanding of some aspects of the systems and/or methods discussed herein. This summary is not an extensive overview of the systems and/or methods discussed herein. It is not intended to identify key/critical elements or to delineate the scope of such systems and/or methods. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention may take physical form in certain parts and arrangement of parts, embodiments of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein:

[0016] FIG. 1 shows a schematic view of a deposition system according to an illustrative embodiment;

[0017] FIG. 2 shows illustrative arrangements of intrinsic and extrinsic microcrystalline layers for single-junction and multi-junction solar cells;

[0018] FIG. 3 is a flow diagram schematically depicting an automated method of depositing a semiconductor material onto a substrate; and

[0019] FIG. 4 is a flow diagram schematically depicting a general method of depositing a semiconductor material onto a substrate.

DETAILED DESCRIPTION

[0020] Certain terminology is used herein for convenience only and is not to be taken as a limitation on the present invention. Relative language used herein is best understood with reference to the drawings, in which like numerals are used to identify like or similar items. Further, in the drawings, certain features may be shown in somewhat schematic form.

[0021] It is also to be noted that the phrase "at least one of, if used herein, followed by a plurality of members herein means one of the members, or a combination of more than one of the members. For example, the phrase "at least one of a first widget and a second widget" means in the present application: the first widget, the second widget, or the first widget and the second widget. Likewise, "at least one of a first widget, a second widget and a third widget" means in the present application: the first widget, the second widget, the third widget, the first widget and the second widget, the first widget and the third widget, the second widget and the third widget, or the first widget and the second widget and the third widget.

[0022] FIG. 1 shows an illustrative embodiment of a plasma-enhanced chemical vapor deposition ("PECVD") system 10 for producing a photovoltaic cell. As shown, the illustrative embodiment of the deposition system 10 includes a deposition chamber 12 that substantially encloses a reaction space 14, where at least one layer, and optionally a plurality of microcrystalline layers of a semiconductor material are to be deposited onto a substrate 16. An example of a physical arrangement of such a deposition chamber 12 can be found in the model KAI-1200 deposition reactor, from Oerlikon Solar AG, of Triibbach, Switzerland. If a dopant is deposited as part of the microcrystalline layer as described in detail below, the resulting microcrystalline layer is referred to as an n-type or p-type doped microcrystalline layer. Microcrystalline layers deposited without a dopant are said to be intrinsic microcrystalline layers.

[0023] A pedestal or other suitable substrate support 18 supports the substrate 16 at a location within the reaction space 14 suitable for deposition. A substrate conditioner 20 can be provided adjacent to the substrate support 18 to adjust and substantially maintain the temperature of the substrate 16 at a desired level for the deposition of the semiconductor material onto the substrate 16. The substrate conditioner 20 can be operable to heat the substrate 16, cool the substrate 16, or both heat and cool the substrate 16 during the deposition described herein. For heating purposes the substrate conditioner 20 can include a heating element that generates thermal energy in any manner such as resistive heating, inductive heating, radiant heating, and the like. The thermal energy required for providing a heating effect to the substrate 16 can optionally be provided, at least in part, by the plasma generated as described herein. For embodiments where a cooling effect is to be provided to the substrate 16, the substrate conditioner 20 can include portions of a refrigeration circuit that removes thermal energy owing at least in part to phase changes of a refrigerant, a conduit that transports a coolant at a lower temperature than the substrate 16 to remove thermal energy from the substrate 16, or any other suitable apparatus for providing the desired cooling effect to the substrate 16 during deposition. The desired temperature of the substrate 16 as established by the substrate conditioner 20 can depend on the particular semiconductor material to be deposited, as well as other process conditions. However, according to the present embodiment, the desired temperature can be any temperature, including any sub-range of temperatures, within a range from about 120°C to about 280°C. According to alternate embodiments the range of temperatures in which the desired temperature falls is from about 140°C to about 220°C, preferably 180°C to about 200°C.

[0024] According to the illustrated embodiment, the substrate support 18 is formed, at least in part, from a metal, metal alloy or other suitable electrically-conductive material to form a first electrode that opposes a second electrode 22. The second electrode 22 is substantially parallel to the substrate support 18 which, according to the present embodiment is also the first electrode, and is separated from the substrate support 18 by a distance D that is normal to the substrate support 18 and the second electrode 22. For various embodiments, the distance D separating the first electrode/substrate support 18 and the second electrode 22 can be greater than or equal to about 10 mm and less than or equal to about 30 mm, although other values of the distance D are also within the scope of the present disclosure. Although the substrate support 18 is the first electrode in the embodiment shown and described with reference to FIG. 1, other embodiments can optionally include a separate first electrode that is distinct from the substrate support 18.

[0025] The first electrode/substrate support 18 and the second electrode 22 in FIG. 1 are operatively connected to a power source 24 for igniting a plasma 26 and maintaining the plasma 26 within the reaction space 14 during at least a portion of the deposition of the semiconductor material onto the substrate 16. For the embodiment shown in FIG. 1, the power source 24 includes a RF generator that can supply RF power having a frequency that is greater than or equal to 13.56 MHz or harmonics of it, such as about 28MHz or 40 MHz, or any other suitable frequency for example. For alternate embodiments, at least one of the first electrode/substrate support 18 and the second electrode has a substantially-planar surface 28 facing the opposite electrode, and comprising a predetermined surface area. The surface area of the planar surface 28 can be selected in conjunction with the power source 24 to establish a desired power density for the particular deposition to be performed. For instance, the surface area of the planar surface 28 of the second electrode 22 and the RF generator can collectively establish a power density that is greater than or equal to 0.1 W per cm² of surface area of the second electrode 22.

[0026] A vacuum subsystem 29 can also be provided to establish a sub- atmospheric pressure within the reaction space 14. The vacuum subsystem 29 can include any device that is operable to at least partially evacuate the deposition chamber 12 to lower the pressure within the reaction space 14 to less than 1 atmosphere. For example, the vacuum subsystem 29 can be operated in cooperation with a process gas delivery subsystem 30 introducing a process gas to the reaction space 14 to maintain the sub-atmospheric pressure within the reaction space 14 at a desired deposition pressure for at least a portion of the deposition of the semiconductor material. Examples of suitable deposition pressures include any pressure that is less than or equal to:

50 mbar^■ mm

D

where the distance D separating the first and second electrodes 18, 22 is expressed in millimeters (mm). In other words, the pressure, expressed in millibar (mbar), within the reaction space 14 multiplied by the distance D, expressed in millimeters (mm), separating the first electrode/substrate support 18 from the second electrode 22 is less than or equal to about 50 mbar*mm.

[0027] The delivery system 30 includes a flow regulator 32, which can be any adjustable device such as a valve, for example, that can regulate, and optionally meter entry of a process gas into the reaction space 14. In addition to the flow regulator 32, the delivery system 30 can also optionally include a mixer 50 that defines a volume in which the components of the process gas can be combined prior to being introduced to the reaction space 14. Individual valves 52 or other flow regulator for each of the various sources 34, 36, 38 can also optionally be disposed along plumbing that establishes fluid communication between the various sources 34, 36, 38 and the reaction space 14. The individual valves 52, if present, can be adjusted to regulate the flow rate of the semiconductor-containing gas, diluting agent and dopant introduced into the reaction space 14. [0028] The process gas can include at least one of: a semiconductor-containing gas from a semiconductor source 34, a diluting agent from a dilution source 36, and a dopant from a dopant source 38. The semiconductor-containing gas can be any gas that includes a semiconducting substance such as silane (S1H4), for example, which includes silicon. The most common diluting agent for the purpose of semiconductor deposition in photovoltaic applications is hydrogen, although any other suitable diluting agent for diluting the concentration of the semiconductor-containing gas is also within the scope of the present disclosure. The dopant includes a material that, when deposited, affects the electrical conductivity of the deposited layer of a semiconducting material. Examples of the dopant include, but are not limited to Phosphine (PH3), Diborane (B₂¾), and

Trimethylboron (B(CH₃)3). For the sake of brevity, and to clearly describe the present technology, the embodiment shown in FIG. 1 and described below includes silane (S1H₄) as the semiconductor-containing gas, and Hydrogen (H₂) as the diluting agent. For depositing N-type microcrystalline layers Phosphine (PH₃) is described as the dopant and, for depositing p-type microcrystalline layers, Doborane (B₂H₆) is used as the dopant in the illustrative embodiments, but again, other suitable P-type and N-type dopants are within the scope of the present disclosure.

[0029] A controller 40 is provided to control operation of at least one of: the power source 24 in supplying RF power to the first electrode/substrate support 18 and second electrode 22 to ignite and maintain the plasma 26, the vacuum subsystem 29 in evacuating at least a portion of the deposition chamber's contents from the reaction space 14, and the delivery subsystem 30 in introducing the process gas to the reaction space 14. The controller 40 can be a microprocessor-based embedded system with access to a non- transitory, computer-readable memory 42, for example. According to such an

embodiment, computer-executable instructions stored in the computer-readable memory 42 can be executed by a microprocessor 46 which, in turn, emits control signals via control lines 44 to portions of the delivery subsystem 30, vacuum subsystem 29 and power supply 24 to be controlled by the controller 44.

[0030] According to alternate embodiments, the controller 40 can be hardwired to perform the various control steps regulating operation of the delivery subsystem 30, vacuum subsystem 29 and power supply 24. For instance, the controller 40 can include one or more application-specific integrated circuits.

[0031] FIG. 2 is a schematic representation of a single-junction solar cell 60 and a multi-junction (dual-junction in the present example) solar cell 62 arranged side- by-side on a common glass substrate 16. As shown, the single-junction solar cell 60 includes a P-type microcrystalline layer 64, on which is deposited an intrinsic

microcrystalline layer 66, followed by an N-type microcrystalline layer 68. A front contact 70 and a back contact 72 made from an electrically-conductive material form the terminals of the single-junction solar cell 60 through which a DC current is generated in response to the single-junction cell 60 being exposed to light 74. The front contact 70 is substantially transparent to transmit most of the light 74 imparted on the front contact 70 to the microcrystalline layers of the semiconductor.

[0032] The doped microcrystalline layer 64 is a P-type layer since it includes atoms that have at least one fewer valence electrons than the semiconductor material deposited to form the microcrystalline material. For the present example where a P-type extrinsic microcrystalline layer 64 of silicon is deposited from silane (SiH₄) as the semiconductor-containing gas, a dopant containing boron, for example, can be introduced to the reaction space 14. Diborane (B₂H₆) and trimethylboron (B(CH₃)3) mentioned above are two examples of suitable dopants for depositing the P-type extrinsic microcrystalline layer 64.

[0033] Similarly, the doped microcrystalline layer 68 is an N-type since it is negatively-doped to include atoms that have at least one more valence electron than the semiconductor material deposited to form the microcrystalline layer. For the present example where the microcrystalline layer 68 is made of silicon as the semiconductor material deposited from silane (SiH₄), a dopant containing phosphorous, for example, can be introduced to the reaction space 14. Phosphine (PH₃), mentioned above, is an example of a suitable dopant for depositing the N-type extrinsic microcrystalline layer 68.

[0034] The intrinsic layer 66 between the P-type microcrystalline layer 64 and the N-type microcrystalline layer 68 is a deposited layer of silicon that has not been intentionally doped during deposition. Thus, the electric conductivity of the intrinsic layer 66 is not altered by the introduction of a dopant.

[0035] The multi-junction solar cell 62 appearing in FIG. 2 is similar to the single-junction solar cell 60, but includes a plurality of repeating stacks comprising P- type, I-type (intrinsic) and N-type layers.

[0036] Depending on the degree of crystallinity of the I-type layer, solar cells such as those appearing in FIG. 2 are characterized as amorphous (a-Si) or

microcrystalline ^c-Si) photovoltaic cells. Microcrystalline layers, as used herein, refer to layers comprising a significant fraction of crystalline silicon - so called micro- crystallites - in an amorphous matrix.

[0037] The controller 40 (FIG. 1) can execute the computer-executable instructions in the memory 42 to perform a method of depositing a semiconductor material onto a substrate 16 disposed in the deposition chamber 12. According to other embodiments of the controller 40, the controller 40 can be hard wired to perform such a method, or some or all of the method steps can be performed manually without departing from the scope of the present application. Illustrative embodiments of such automated methods can be understood with reference to the flow diagrams appearing in FIG. 3. The order in which the steps appear in FIG. 3 is not necessarily the order in which the steps are required to be performed unless specified otherwise.

[0038] One example of the PECVD process is schematically depicted in FIG. 3 for depositing intrinsic and/or doped microcrystalline-silicon layers using a standard deposition machine such as the model KAI-1200 deposition system commercially available from Oerlikon Solar AG. This example will be described with silane (SiH₄) as the semiconductor-containing gas, which is present at mixture fractions above fifty (50%) percent of the total process gas within the reaction space 14. The diluting agent in the present example includes hydrogen (H₂), and for embodiments where a doped

microcrystalline layer is to be deposited, a dopant such as phosphine (PH ), diborane (B₂H₆), or trimethylboron (B(CH₃)₃) can be added. The flow rate of the process gas being introduced to the reaction space 14 during deposition is low in the present example, less than 0.03 sccm/cm² of surface area of the planar surface 28 (FIG. 1) of the second electrode 22. Further, the sub-atmospheric pressure within the deposition chamber in the present example is maintained at or below normalized pressures of 50 mbar*mm

(pressure*distance separating electrodes). Results of the deposition yielded deposition rates of at least 5 A/s (50 nm/s) at high RF power densities (i.e., greater than 0.1 W/cm² of surface area of the planar surface 28 (FIG. 1 ) of the second electrode 22). The above parameter specifications for the present example have been normalized based on one cm² surface area of the planar surface 28 for standardized scaling to other suitable deposition systems 10.

[0039] As shown in FIG. 3, the method includes receiving, with the controller 40, a sub-atmospheric pressure to be established within the deposition chamber 12 during at least a portion of the deposition of the semiconductor material at step 100. The sub- atmospheric pressure can be programmed into the controller 40 during construction of the deposition system 10, input by a user operating the deposition system 10, or otherwise input to the controller 40. Regardless of the way the sub-atmospheric pressure is specified, the sub-atmospheric received can be less than or equal to:

50 mbar · mm

D

where the distance D separating the first and second electrodes is expressed in millimeters (mm). For the present example, the distance D separating the first and second electrodes is greater than or equal to about 10 mm and less than or equal to about 30 mm. For alternate embodiments, the sub-atmospheric pressure received by the controller 40 at step 100 is at least 0.8 mbar, but not greater than 3.0 mbar. Yet other embodiments require the sub-atmospheric pressure received by the controller 40 at step 100 to be at least 1.0 mbar, but not greater than 2.0 mbar. The controller 40 can subsequently transmit a pressure signal to be delivered along control lines 44 that controls operation of the vacuum subsystem 29 at step 1 10 in FIG. 3 to at least partially evacuate the deposition chamber 12 and establish the sub-atmospheric pressure received.

[0040] Similarly, the controller 40 also receives a target temperature of the substrate 16 for the deposition process to be performed at step 120. Just as with the sub- atmospheric pressure, the target temperature can be programmed into the controller 40 during construction of the deposition system 10, input by a user operating the deposition system 10, or otherwise input to the controller 40. Regardless of the way the target temperature is specified, the target temperature received for the present example is at least 120°C, but not greater than 280°C. According to alternate embodiments, the received target temperature is at least 140°C but not greater than 220°C, and preferably from about 180°C to about 200°C. The controller 40 subsequently transmits a temperature signal to be delivered along the control lines 44 (FIG. 1) to control operation of the substrate conditioner 20 at step 130 in FIG. 3 to elevate or lower the temperature of the substrate 16 to a temperature approaching the received target temperature.

According to one embodiment, the delivery system 30 (FIG. 1) can optionally introduce an ignition gas at step 140 in FIG. 3 to the reaction space 14 prior to introduction of the process gas, and prior to ignition of the plasma 26. The ignition gas can be a gas such as hydrogen (H₂) from the dilution source 36 (FIG. 1) or an inert gas, for example. Following introduction of the optional ignition gas, the controller 40 can transmit a plasma signal at step 150 that causes the power source to energize the first and second electrodes 18, 22 and establish the plasma 26 within the deposition chamber 12 in the presence of the ignition gas. According to the present example, the power source comprises a RF generator that provides RF power having a frequency that is at least 13.56 MHz or a harmonic of that frequency, such as about 28MHz or 40 MHz, for example. According to alternate embodiments, the frequency can be at least 35 MHz, or at least 40 MHz. Further, the RF power supplied comprises a power density that is greater than or equal to 0.1 W per cm of the surface area of a planar surface 28 of the second electrode 22.

[0041] Subsequent to ignition of the plasma 26 (FIG. 1), the controller 40 transmits a flow signal via control lines 44 at step 160 that controls operation of the delivery system 30 to introduce the process gas comprising at least silane (SiH₄) and a suitable quantity of hydrogen (H₂) into the deposition chamber 12. Operation of the delivery system 30 establishes a concentration of silane (SiH₄) in the reaction space 14 of at least fifty (50%) percent by volume during at least a portion of the deposition, and possibly during a majority or all of the deposition. For deposition of intrinsic

microcrystalline layers, the concentration of silane (SiH₄) can be at least seventy (70%) percent by volume, or at least seventy-five (75%) percent by volume. Regardless of the concentration of silane (S1H₄), the controller can be further programmed to adjust a portion of the delivery subsystem 30 to establish a flow rate for the process gas being introduced to the reaction space 14 of about 0.03 seem per cm² of surface area A of the planar surface 28 of the first and/or second electrodes 18, 22. Deposition of

microcrystalline layers according to such a method yield growth rates of at least 5 A/sec (50 nm/s).

[0042] The method described with reference to FIG. 3 is an example of an automated method. As mentioned above, however, one or more steps can be performed manually or by means other than the controller 40, without departing from the scope of the application. So regardless of the entity performing such steps, the general method of controlling deposition of a semiconductor material described herein using the deposition system 10 can be understood with reference to FIG. 4.

[0043] As depicted in FIG. 4, a sub-atmospheric pressure is established at step 200 within the deposition chamber 12, and the sub-atmospheric pressure is maintained during at least a portion of the deposition of the semiconductor material. Just as before, the sub-atmospheric can be less than or equal to:

50 mbar mm

D

wherein the distance D separating the first and second electrodes is expressed in mm. For the present example, the distance D separating the first and second electrodes is at least about 10 mm, but not greater than about 30 mm. For alternate embodiments, the sub- atmospheric pressure to be established is at least 0.8 mbar, but not greater than 3.0 mbar. Yet other embodiments require the sub-atmospheric pressure established at step 200 to be at least 1.0 mbar, but not greater than 2.0 mbar. Regardless of its value, the sub- atmospheric pressure can be established by controlling operation of at least one of the vacuum subsystem 29 and the delivery subsystem 30.

[0044] Using the substrate conditioner 20 (FIG. 1), a temperature of the substrate 16 is adjusted (i.e., elevated, lowered or maintained) at step 210 of FIG. 4 to a temperature approaching, or approximately equal to a target temperature for the particular deposition process being performed. As described above, the target temperature can be programmed into the controller 40, input by an operator via a control panel, or otherwise specified. For the present example using silane (S1H₄) as the semiconductor-containing gas, the target temperature is at least 120°C, but not greater than 280°C. According to alternate embodiments, the target temperature is at least 140°C, but not greater than 220°C, and preferably from about 180°C to about 200°C.

[0045] According to one embodiment, the delivery system 30 (FIG. 1) can optionally introduce the ignition gas at step 220 in FIG. 4 to the reaction space 14 prior to introduction of the silane (S1H₄) or other semiconductor-containing gas, and prior to ignition of the plasma 26. The ignition gas can be a gas such as hydrogen (H₂) or an inert gas from the dilution source 36 (FIG. 1), for example, that does not deposit a significant quantity of, or optionally any unwanted solids in the presence of the plasma 26, once ignited.

[0046] Following the optional introduction of the ignition gas, the power source 24 (FIG. 1) is used at step 230 to energize the first and second electrodes 18, 22 to establish the plasma 26 within the deposition chamber 12, optionally in the presence of the ignition gas. According to the present example, the power source comprises a RF generator that provides RF power having a frequency of 13.56 MHz or a harmonic of that frequency, such as about 28MHz or 40 MHz, for example. According to alternate embodiments, the frequency can be at least 35 MHz, or at least 40 MHz. Further, the RF power supplied comprises a power density that is greater than or equal to 0.1 W per cm of the surface area of a planar surface 28 of the first and/or second electrodes 18, 22.

[0047] Subsequent to ignition of the plasma 26 (FIG. 1), and in the presence of the plasma 26, a portion of the delivery subsystem 30 (such as the flow regulator 32) is adjusted at step 240 to introduce the process gas comprising at least silane (S1H₄) and a suitable quantity of hydrogen (H₂) into the deposition chamber 12. Operation of the delivery subsystem 30 establishes a concentration of silane (S1H₄) in the reaction space 14 of at least fifty (50%) percent by volume during the deposition. For deposition of intrinsic microcrystalline layers, the concentration of silane (S1H₄) can be at least seventy (70%) percent by volume, or at least seventy-five (75%) percent by volume, diluted by hydrogen (H₂) as the diluting agent. For deposition of an extrinsic microcrystalline layer, the concentration of silane (SiH₄) in the reaction space 14 can be established as at least fifty (50%) percent by volume during the deposition, and the composition of the combination of the dopant and hydrogen (H₂) can be at least 30%. For such

embodiments, the combination of the dopant and hydrogen (H₂) can include a dopant concentration of less than 1% by volume, diluted with the hydrogen (H₂). Regardless of the concentration of silane (SiH₄), the delivery subsystem 30 can be controlled to establish a flow rate for the process gas being introduced to the reaction space 14 of about 0.03 seem per cm² of surface area A of the planar surface 28 of the first and/or second electrodes 18, 22. According to other embodiments, the total flow rate of the process gas introduced to the reaction space 14 can be maintained at less than 500 seem.

[0048] The following deposition examples have been performed in accordance with the method and system described herein.

EXAMPLE #1

[0049] Deposition of an intrinsic microcrystalline silicon layer using a deposition model KAI-1200 from Oerlikon Solar AG:

[0050] The process gas comprised about 75% SiH₄ and about 25% H₂.

[0051] Total flow rate of the process gas introduced to the reaction space during deposition was about 2.5 sccm/(100cm of surface area of the planar electrode surface 28), which for the present example amounted to a process gas volumetric flow rate of about 330 seem SiH₄ and about 100 seem ¾.

[0052] The energy supplied by the power source 24 was RF power that had a frequency of about 40 MHz RF frequency and a power density of about 0.17 W/(cm of surface area of the planar electrode surface 28), which amounted to about 3,000 W per deposition chamber in the present example.

[0053] The pressure within the reaction space was maintained at

approximately 1.3 mbar, for a distance D of about 28 mm separating the first and second electrodes 18, 22 (i.e., about 36.4 mbar*mm). [0054] The substrate temperature was maintained between 120°C and 280°C during deposition.

[0055] In comparison to conventional deposition processes where highly- diluted silane (SiH₄) (i.e., process gas that includes silane concentrations of less than 10% by volume diluted with about 90% hydrogen by volume), hydrogen (H₂) consumption was reduced by about 95%, silane (S1H₄) usage efficiency increased by approximately 35%, and the growth rate of the microcrystalline layer increased by approximately 35% over such conventional deposition processes.

EXAMPLE #2

[0056] Deposition of an N-type, extrinsic microcrystalline silicon layer using a deposition model KAI-1200 from Oerlikon Solar AG:

[0057] The process gas comprised about 67% SiH₄ and about 33% dopant gas comprising phosphine (PH₃), wherein the dopant gas comprised approximately 0.5% phosphine (PH₃) by volume diluted in hydrogen (H₂).

[0058] The total flow rate of the process gas introduced to the reaction space 14 during deposition was about 2.5 sccm/( 100cm² of surface area of the planar electrode surface 28), which for the present example amounted to a process gas volumetric flow rate of about 300 seem S1H4 and about 150 seem of the dopant gas.

[0059] The energy supplied by the power source 24 was RF power that had a frequency of about 40 MHz RF frequency and a power density of about 0.2 W/(cm of surface area of the planar electrode surface 28), which amounted to about 3,500 W per deposition chamber 12 in the present example.

[0060] The pressure within the reaction space was maintained at

approximately 1.3 mbar, for a distance D of about 28 mm separating the first and second electrodes 18, 22 (i.e., about 36.4 mbar*mm).

[0061] The substrate temperature was maintained between 120°C and 280°C during deposition.

[0062] In comparison to conventional deposition processes where highly- diluted silane (S1H₄) (i.e., process gas that includes silane concentrations of less than 10% by volumen diluted with about 90% hydrogen by volume), hydrogen (H₂) consumption was reduced by about 95%, silane (S1H₄) usage efficiency increased by approximately 35%, and the growth rate of the microcrystalline layer increased by approximately 35% over such conventional deposition processes.

[0063] Illustrative embodiments have been described, hereinabove. It will be apparent to those skilled in the art that the above devices and methods may incorporate changes and modifications without departing from the general scope of this invention. It is intended to include all such modifications and alterations within the scope of the present invention. Furthermore, to the extent that the term "includes" is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term "comprising" as "comprising" is interpreted when employed as a transitional word in a claim.

Claims

CLAIMS What is claimed is:

1. A deposition system for producing a photovoltaic cell, the deposition system comprising:

a deposition chamber that substantially encloses a reaction space where a semiconductor material is to be deposited onto a substrate to form a microcrystalline layer of the semiconductor material on the substrate;

a substrate support that supports the substrate in the reaction space;

first and second electrodes that oppose each other and are separated by a distance D, the first and second electrodes being operatively connected to a power source to be energized for igniting a plasma and maintaining the plasma in the reaction space during at least a portion of the deposition;

a vacuum subsystem that at least partially evacuates the deposition chamber; a delivery subsystem that introduces a process gas to the reaction space, the process gas comprising: a semiconductor-containing gas from a semiconductor source and a diluting agent from a dilution source; and

a controller programmed to control operation of at least one of the vacuum subsystem and the delivery subsystem to:

maintain the sub-atmospheric pressure during at least a portion of the deposition of the semiconductor material at a pressure that is less than or equal to:

50 mbar mm

D

where the distance D separating the first and second electrodes is expressed in mm, and establish a concentration of the semiconductor-containing gas in the process gas of at least fifty (50%) percent by volume during at least a portion of the deposition of the semiconductor material.

2. The deposition system according to claim 1 , further comprising a substrate conditioner that provides a heating effect to the substrate or provides a cooling effect to the substrate, or provides heating and cooling effects to the substrate to establish a desired temperature for deposition of the semiconductor material onto the substrate.

3. The deposition system of claims 1-2, wherein the first electrode comprises the substrate support.

4. The deposition system of claims 1-3, wherein the desired temperature of the substrate established by the substrate conditioner is within a range of temperatures from about 120°C to about 280°C.

5. The deposition system of claims 1-4 , wherein the range of temperatures is from about 140°C to about 220°C.

6. The deposition system of claims 1-5, wherein the power source comprises a RF generator that provides RF power having a frequency that is greater than or equal to 35 MHz.

7. The deposition system of claim 6, wherein at least one of the first and second electrodes comprises a substantially planar surface comprising a surface area A, and the RF power comprises a power density that is greater than or equal to 0.1 W per cm of the surface area A.

8. The deposition system of claims 1-7, wherein the distance D separating the first and second electrodes is greater than or equal to about 10 mm and less than or equal to about 30 mm.

9. The deposition system of claim 1-8, wherein at least one of the first and second electrodes comprises a substantially planar surface comprising a surface area A, and the controller is further programmed to establish a flow rate for the process gas being introduced to the reaction space of about 0.03 seem per cm² of the surface area A.

10. The deposition system of claims 1-9, wherein the controller is further programmed to maintain the sub-atmospheric pressure at a pressure that is greater than or equal to 0.8 mbar and less than or equal to 3.0 mbar.

1 1. The deposition system of claims 1-10, wherein the controller is programmed to maintain the concentration of the semiconductor-containing gas in the process gas above seventy (70%) percent by volume during the portion of the deposition of the semiconductor material.

12. The deposition system of claims 1-1 1, wherein the process gas comprises about seventy five (75%) percent by volume silane (SiH₄) as the semiconductor- containing gas and about twenty five (35%) percent by volume hydrogen (H₂) as the diluting agent.

13. The deposition system of claims 1-12, wherein the delivery subsystem introduces a dopant in combination with the diluting agent, wherein the dopant comprises an impurity that is to be included in the microcrystalline layer to establish a doped microcrystalline layer.

14. A method of depositing a semiconductor material onto a substrate in a deposition chamber of a deposition system, the deposition system further comprising first and second electrodes separated by a distance D and operatively connected to a power source for establishing a plasma in the deposition chamber, a vacuum subsystem for at least partially evacuating the deposition chamber, and a delivery subsystem for introducing a process gas to the deposition chamber, the method comprising:

with a controller, receiving a sub-atmospheric pressure to be established within the deposition chamber during at least a portion of the deposition of the semiconductor material, the sub-atmospheric pressure being less than or equal to:

50 mbar^■ mm

D

wherein the distance D separating the first and second electrodes is expressed in mm; transmitting a pressure signal that controls operation of the vacuum subsystem to at least partially evacuate the deposition chamber and establish the sub-atmospheric pressure received;

with the controller, transmitting a plasma signal that controls the power source to energize the first and second electrodes and establish the plasma within the deposition chamber; and

transmitting a flow signal that controls operation of a delivery subsystem to introduce a semiconductor-containing gas and a suitable quantity of a diluting agent into the deposition chamber to establish a concentration of the semiconductor-containing gas within the deposition chamber of at least fifty (50%) percent by volume during at least a portion of the deposition.

15. The method of claim 14, the deposition system further comprising a substrate conditioner that provides one or both of a heating effect and a cooling effect to the substrate;

with the controller, receiving a target temperature of the substrate for the deposition; transmitting a temperature signal that controls the substrate conditioner to elevate, lower or both elevate and lower a temperature of the substrate to a temperature approaching or approximately equal to the target temperature;

16. The method of claims 14-15, wherein the target temperature received is at least 120°C and not greater than 280°C.

17. The method of claims 14-16, wherein the target temperature received is at least 140°C and not greater than 220°C.

18. The method of claims 14-17, wherein at least one of the first and second electrodes comprises a substantially planar surface comprising a surface area A, and the flow signal comprises an instruction for adjustment of the flow regulator to establish a desired flow rate of the process gas being introduced to the reaction space of about 0.03 seem per cm² of the surface area A.

19. The method of claims 14-18, wherein the pressure signal comprises an instruction to maintain the sub-atmospheric pressure within a range from 0.8 mbar and less than or equal to 3.0 mbar.

20. The method of claims 14-19, further comprising transmitting a dopant signal that controls operation of the delivery subsystem to introduce a dopant in combination with the diluting agent, wherein the dopant comprises an impurity that alters an intrinsic electrical conductivity of the microcrystalline layer to establish a doped microcrystalline layer.