One embodiment provides an apparatus for material deposition. The apparatus includes a reaction chamber, and a pair of susceptors. Each susceptor has a front side and a back side, and the front side mounts substrates. The susceptors are positioned vertically in such a way that the front sides of the susceptors face each other, and the vertical edges of the susceptors are in contact with each other, thereby forming a substantially enclosed narrow channel between the substrates. The apparatus also includes a number of gas nozzles for injecting reaction gases. The gas nozzles are controlled in such a way that gas flow directions inside the chamber can be alternated, thereby facilitating uniform material deposition. The apparatus includes a number of heating units situated outside the reaction chamber. The heating units are arranged in such a way that they radiate heat energy directly to the back sides of the susceptors.

BACKGROUND

This disclosure is generally related to silicon deposition. More specifically, this disclosure is related to a scalable, high throughput multi-chamber batch type epitaxial reactor for silicon deposition.

2. Related Art

The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.

A solar cell converts light into electricity using the photoelectric effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer of similar material. A hetero-junction structure includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an optional intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi-junction structure includes multiple semiconductor layers of different bandgaps stacked on top of one another.

In a solar cell, light is absorbed near the p-n junction generating carries. The carries diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.

Based on industrial surveys, crystalline-Si-wafer based solar cells dominate nearly 90% of the market. However, the cost of producing crystalline-Si-wafer based solar cell is high, and the waste of Si material in the processes of ingot-cutting and wafer-polishing has caused a bottleneck in the supply of crystalline-Si wafers. Due to the soaring price and the supply shortage of Si material, there has been a great interest in alternative ways to make solar cells. Recently, photovoltaic thin-film technology has been drawing vast interest because it can significantly reduce the amount of material used and thus lower the cost of solar cells. Among various competing technologies, single-crystal Si thin-film solar cells have drawn great interest for their low cost and high efficiency.

Single-crystal Si thin-film solar cells can be created using conventional semiconductor epitaxy technologies which not only reduce manufacturing costs but also enable flexible doping levels in the emitter, absorber and back surface field of the solar cell, thus enhancing its efficiency. Single-crystal Si thin-film solar cells with an efficiency as high as 17% have been demonstrated in research labs (see M. Reutuer et al., “17% Efficient 50 μm Thick Solar Cells,”Technical Digest,17thInternational Photovoltaic Science and Engineering Conference,Fukuoka, Japan, p. 424).

A high-quality single-crystal Si thin film can be produced using Si epitaxy, which has been widely used in semiconductor industry to create a high-quality single-crystal Si layer for CMOS integrated circuits, power devices and high voltage discrete devices. Among possible Si epitaxial deposition techniques, trichlorosilane (TCS) based chemical-vapor-deposition (CVD) can provide a deposition rate up to 10 μm/min. Therefore, it is possible to achieve a high-throughput and low-cost epitaxial process for solar cell application.

However, there is a lack of suitable Si epitaxy tools that can meet the demand for high throughput and low deposition cost for Si film layers with thickness up to several tens of microns, as required by the solar cell industry. Existing Si epitaxy tools, such as AMC7810™ and Centura 5200™ by Applied Materials Inc. of Santa Clara, Calif., US; MT7700™ by Moore Epitaxial Inc. of Tracy, Calif., US; PE2061™ by LPE Epitaxial Technology of Italy; and Epsilon 3200™ by ASM International of the Netherlands, are optimized for the needs of semiconductor device manufacturing. Although these epitaxial tools can deliver Si films with the highest quality, these tools are not compatible, in terms of throughput and gas conversion efficiency, with the economics of the solar cell industry.

FIG. 2presents a diagram illustrating the structure of an existing barrel epitaxial reactor, which is used for the batch process of multiple wafers. Barrel reactor200includes a reaction chamber202, which has a gas inlet204at the top and a vent206at the bottom. A vertically positioned susceptor208holds a number of wafers, such as wafer210. Radio frequency (RF) heating coils212radiate heat onto the susceptor and wafers. Although barrel reactor200can batch process multiple wafers, the number of wafers it can process is limited by the architect of the system, the size of the chamber, and the design of the susceptor. Once built, it is difficult to modify the reactor or the susceptor to accommodate more wafers. In addition, the susceptor needs to be rotated during deposition in order to achieve a better uniformity.

U.S. Pat. No. 6,399,510 proposed a reaction chamber that provides a bi-directional process gas flow to increase uniformity without the need for rotating susceptors. However, it does not solve the issues of low throughput, low reaction gas conversion rate, low power utilization efficiency, minimal Si deposition on the quartz chamber, and processing scalability. In addition, using the same gas lines for gas inlet and outlet increased the risk of contamination and re-deposition.

SUMMARY

One embodiment of the present invention provides a system for material deposition. The system includes an AC (alternating current) panel for providing electrical power to the system, a susceptor load/unload station, a running beam coupled to the load/unload station for loading/unloading susceptors, and a multi-chamber module. The multi-chamber module includes a gas box, an SCR panel, and a number of reaction chambers situated next to each other. The reaction chamber is formed using a material that is transparent to radiation energy, a pair of susceptors situated inside the reaction chamber. Each susceptor has a front side and a back side, and the front side mounts a number of substrates. The susceptors are positioned vertically in such a way that the front sides of the susceptors face each other, and the vertical edges of the susceptors are in contact with each other, thereby forming a substantially enclosed narrow channel between the substrates mounted on different susceptors. The system also includes a number of gas nozzles. At least one of the gas nozzles includes a gas inlet for injecting reaction gas into the narrow channel and a gas outlet for outputting exhaust. The gas inlet and the gas outlet are coupled to different gas lines, and the gas inlet and the gas outlet are controlled in such a way that reaction gas flow directions inside the narrow channel can be alternated, thereby facilitating uniform material deposition. In addition, the system includes a number of heating units situated outside the reaction chamber. At least one heating unit is situated between the side walls of two adjacent reaction chambers, thereby allowing the at least one heating unit to heat the two adjacent reaction chambers simultaneously. In addition, the heating units are arranged in such a way that they radiate heat energy directly to the back side of the susceptors.

In a variation on the embodiment, the susceptors are formed using SiC-coated graphite or monolithic SiC.

In a variation on the embodiment, the cross section of the susceptors are shaped as a “U,” and the wafer-holding sides of the susceptors are the inner surfaces of the “U.”

In a variation on the embodiment, the reaction gas includes at least one of the following: SiH4, SiH2Cl2, SiHCl3, and SiCl4.

In a variation on the embodiment, the gas inlet is configured to inject a small amount of purge gas when the gas inlet is not injecting reaction gas to the narrow channel during material deposition, thereby preventing material deposition around the gas inlet.

In a variation on the embodiment, the width of the narrow channel is between 5 mm and 200 mm, preferably between 20 mm and 30 mm.

In a variation on the embodiment, the system includes a number of gas nozzles for injecting purge gas between the back side of the susceptors and the inner walls of the reaction chamber.

In a variation on the embodiment, the system includes a closed-loop feedback control for controlling the number and power of heating units.

In a variation on the embodiment, the multi-chamber module can be placed adjacent to at least one more multi-chamber module, and wherein the multi-chamber modules share same power supply and gas source.

DETAILED DESCRIPTION

Overview

Embodiments of the present invention provide a scalable, high-throughput multi-chamber epitaxial reactor for Si deposition. The reactor includes a number of extendible, independently controlled multi-chamber modules. The reaction chambers are heated by lamp heating units which are alternately inserted between adjacent chambers. Each reaction chamber encloses a pair of susceptors for supporting substrates. Reaction gases are injected into the chamber from one side to another alternatively to ensure deposition uniformity.

FIG. 3presents a block diagram illustrating the side view of a 9-chamber epitaxial reactor in accordance with an embodiment. The back portion (left side ofFIG. 3) of the reactor includes gas/chemical sources, such as a gas/chemical box302, and various control panels, such as an AC (alternating current) panel304and an SCR (silicon-controlled rectifier) panel306. A three-zone heat exchanger and blower308resides beneath AC panel304. The front portion of the reactor includes a number of reaction chambers310. Each reaction chamber is surrounded by lamp heating units314and covered with a lid312. A running beam316is attached to the front of the reactor and interfaces with a factory load/unload susceptor station318. A susceptor stand320is situated above load/unload station318.

AC panel304controls the power supply for the entire reactor; gas/chemical box302includes the sources of input gases, such as TCS and H2carrier gas; and SCR panel306controls the operation of lamp heating units that surround the chambers. Details of gas/chemical box302and SCR panel306are shown inFIG. 4, which presents a diagram illustrating the back view of the 9-chamber epitaxial reactor in accordance with an embodiment.

The top portion ofFIG. 4illustrates three gas panels402,404, and406. Each gas panel controls the gas inputs for three individual reaction chambers. For example, gas panel402includes gas controls for three individual chambers430,432, and434. The bottom portion ofFIG. 4illustrates three SCR panels410,412, and414. Each SCR panel controls the lamp heating units surrounding three corresponding chambers. For example, SCR panel410controls lamp heating units surrounding chambers430,432, and434. In addition, each SCR panel has four controls including controls416-422, and each control independently controls a group of lamps. All lamp heating units surrounding the three reaction chambers are divided into four groups depending on their locations. For example, all lamps residing at the top of the chambers are grouped together to be controlled by top control416. Similarly, middle control418, bottom control420, and edge control422control lamps that reside in between the chambers, at the bottom of the chambers, and at the edge of the chambers, respectively. Allowing individual control of lamps located at different locations of a chamber ensures that a uniform temperature can be maintained over a large flat zone inside the reaction chamber and the susceptors can be heated uniformly. In one embodiment, the SCR controls also include a closed-loop feedback mechanism which can further improve the heating uniformity inside the chamber.

FIG. 4demonstrates that the nine reaction chambers are divided into three groups, each including three chambers. Each group has its own gas panel and heating control. For example, chamber430,432, and434forms one group, which has its own gas panel402and SCR panel410for heating controls. The three reaction chambers together with their corresponding gas panel and SCR panel form a multi-chamber module. Because each multi-chamber module can operate independently of other modules, the whole system can provide a flexible throughput. For example, under certain circumstances, only one or two modules of the reaction are operating. In addition, the modular configuration also provides processing scalability. For example, to increase the batch process capability, one can simply add more modules, each including reaction chambers, gas sources, and SCR control, to the existing system without the need to modify the size of the reaction chamber or the configuration of the susceptors. Note that, other than grouping three reaction chambers into one module, other configurations are also possible.

Returning toFIG. 3, three-zone heat exchanger and blower308provides a forced airflow in a plenum330surrounding reaction chamber310. Arrows326indicate the direction of the forced airflow in plenum330. Consequently, a pressurized airflow is maintained along the exterior walls of reaction chamber310to keep the walls at a uniformly cooler temperature relative to the temperature inside the chamber. In one embodiment, the temperature of the outer surface of chamber310is kept at approximately 600° C., thus minimizing Si deposition on the chamber walls.

Before Si deposition, running beam316picks up susceptor322from factory load/unload susceptor station318, which is configurable for automatic guided vehicle (AGV), overhead hoist transport (OHT), or a conveyer transport system. Running beam316then carries susceptor322into a load lock332. A laminar airflow, as shown by arrows328, is maintained in load lock332during loading to repel dust and other impurities. Chamber310's lid312opens in a direction as shown by arrow324, and susceptor322can be dropped inside chamber310for Si deposition. Depending on the configuration of running beam316, one or more susceptors can be loaded inside the chamber each time.

FIG. 5Apresents a diagram illustrating the top view of the base plate and chambers of the 9-chamber epitaxial reactor in accordance with an embodiment. Base plate502can be made of stainless steel or other durable materials. The shape of the cross section of reaction chamber504can be, but is not limited to: oblong, rectangular, circular, square, or other shapes.

FIG. 5Bpresents a diagram illustrating the front view of the chambers in the 9-chamber epitaxial reactor in accordance with an embodiment. Lamp heating units, such as lamp heating units506and508, are alternately inserted between reaction chambers. Consequently, one set of lamp heating units can radiate heat to chambers on both sides, thus greatly increasing energy utilization. For example, lamp heating unit508, which resides between chambers510and512, radiates heat to both chambers. The size of each lamp or the number of lamps in each lamp heating unit can be varied depending on the size of the chamber and the power of the lamp. In one embodiment, each chamber is surrounded by 22 lamps. The heating units can be made of RF heating coils or tungsten lamps. To further increase energy efficiency, in one embodiment, gold-coated reflectors are mounted around the chambers to reflect back most of the radiant energy from the lamps. In comparison to traditional epitaxial reactors, the radiant heat from lamp heating units in present reactor design is efficiently utilized; thus, the electricity consumption of the 9-chamber epitaxial reactor is significantly reduced, resulting in the reduction of the cost of the epitaxy process.

Chamber and Susceptors

FIG. 6Apresents a diagram illustrating the front side of a susceptor inside a reaction chamber in accordance with an embodiment. A susceptor604is placed vertically inside a chamber602. To avoid heat absorption by chamber walls, chamber602is formed using a material that is transparent to radiant heat. In one embodiment, chamber602is formed using quartz. By contrast, susceptor604can be formed using a material that is opaque and absorbs radiant heat energy, such SiC-coated graphite and monolithic SiC. In one embodiment, susceptor604is formed using SiC-coated graphite. As a result, most of the radiant heat from the lamp heating units is absorbed by susceptor604. In addition, the walls of chamber602are kept cool by surrounding forced airflows to reduce Si deposition on the inner surface.

The front side of susceptor604includes a set of pockets, such as pocket606, for supporting substrates to be deposited. The shape of the bottom of the pockets is carefully designed to ensure a good thermal contact between the susceptor and the substrates. In one embodiment, the bottom of pocket606has a contour shape. Depending on the size of susceptor604, various numbers of substrates can fit onto susceptor604. In one embodiment, susceptor604includes 12 pockets for supporting 12 125×125 mm2substrates.

FIG. 6Bpresents a diagram illustrating the side view of a reaction chamber in accordance with an embodiment.FIG. 6Billustrates an outer wall of quartz chamber602.FIG. 6Cpresents a diagram illustrating the front view of the cross section of a reaction chamber in accordance with an embodiment.FIG. 6Cdemonstrates that a pair of susceptors, susceptor604and susceptor608, are placed vertically inside reaction chamber602. A narrow channel610is formed between susceptors604and608. Also shown inFIG. 6Care a number of gas nozzles, including gas nozzles612,614,616, and618.

FIG. 6Dpresents a diagram illustrating the top view of the cross section of a reaction chamber in accordance with an embodiment.FIG. 6Dillustrates that the cross sections of susceptors604and608are shaped like an “U.” The vertical edges of susceptors604and608are in contact with each other forming an enclosed narrow channel610. As a result, during deposition, the precursor gases, such as TCS, can be contained within narrow channel610. Other examples of precursor gases include, but are not limited to: SiH4, SiH2Cl2, and SiCl4. In addition to “U” shape, the cross sections of susceptors604and608can form other shapes, include but are not limited to: half circle, half eclipse, and other regular or irregular shapes. Note that the front sides (i.e., the wafer-holding sides) of susceptors604and608are facing each other. Thus, the deposition substrates, such as substrate606, have their deposition surfaces surround channel610, which contains the precursor gases and keeps them from depositing material on the inner walls of chamber602. Such a configuration can increase the TCS gas utilization rate significantly because the probability for the TCS gas to successfully deposit Si on substrates surfaces is now much higher. The increased deposition probability results from the precursor gases being surrounded by deposition surfaces as well as the reduced deposition on the inner walls of chamber602. Note that channel610cannot be too narrow to ensure sufficient gas flow in the channel. The width of channel610(the distance between susceptors604and608) can be between 5 mm and 200 mm. In one embodiment, the width of channel610is between 20 mm and 30 mm and a TCS utilization rate of up to 30% can be achieved.

In addition to enabling better gas utilization, this configuration has the back sides of the susceptors facing the chamber wall and the lamp heating unit, which ensures efficient radiant-heat-energy absorption from the lamp heating units by the black susceptors. The susceptors then transfer the absorbed heat energy to the substrates. In an alternative embodiment, a single susceptor is placed vertically inside the reaction chamber. Deposition substrates are mounted on both sides of the susceptor and face lamp heating unit directly.

In a solar cell, film uniformity greatly impacts the solar cell's efficiency. In a traditional epitaxial system, it has been difficult to achieve good deposition uniformity and a high reaction-gas-utilization rate at the same time. Substrate rotation can be used to improve uniformity. However, it becomes increasingly difficult to rotate substrates in a large batch reactor. To achieve better deposition uniformity, in one embodiment, precursor gases, such as TCS and H2, are injected into channel610inside chamber602via gas nozzles612and614, which are located at the top and bottom of chamber602, respectively. During deposition, the chamber pressure can be kept between 1 Torr and 1520 Torr.FIG. 6Epresents a diagram illustrating the top view of a gas nozzle in accordance with an embodiment of the present invention. Gas nozzle614includes a gas inlet620and a gas outlet622, which are segregated from each other by a gas ring624. The structure of gas nozzle612is similar to that of gas nozzle614. Gas inlet620is used for injecting precursor gases to channel610and gas outlet622is used for outputting exhaust. Because gas inlet620and gas outlet622are segregated by gas ring624and are coupled to different gas lines, there is no contamination from the exhaust. The gas inlets of nozzles612and614couple to gas sources through a switching manifold valve and are configured to turn on alternately. In addition, the opening and closing of gas outlets of nozzles612and614also alternate to synchronize with the close and open steps of gas inlets. As a result, the gas flow direction inside chamber602, or, more specifically, inside channel610, is alternated in sequence.

FIG. 6Fpresents a diagram illustrating the gas flow sequence of gas inlets and outlets in accordance with an embodiment of the present invention. During step 1, gas inlet630of the top gas nozzle is open to inject precursor gases including TCS and H2into channel610. Arrow638indicates the flow direction of the precursor gases. Also in step 1, gas outlet636of the bottom gas nozzle is open to output exhaust gas. Arrow640indicates the flow direction of the exhaust gas. Gas outlet632of the top gas nozzle and gas inlet634of the bottom gas nozzle are closed during step 1.

Similarly, during step 2, gas inlet634of the bottom gas nozzle is open to inject precursor gases including TCS and H2into channel610. Arrow642indicates the flow direction of the precursor gases. Also in step 2, gas outlet632of the top gas nozzle is open to output exhaust gas from channel610. Arrow644indicates the flow direction of the exhaust gas. Gas inlet630of the top gas nozzle and gas outlet636of the bottom gas nozzle are closed during step 2. Because the current configuration allows the flow direction of the precursor gases inside channel610to alternate sequentially, a uniform deposition characteristic on substrates can be achieved without the need to rotate the susceptors. Note that besides placing gas nozzles at the top and bottom of the chamber, other configurations, such as different numbers of nozzles or different nozzle positions, are also possible for improving uniformity.

In order to prevent Si deposition around gas inlets630and634while they were closed for injection, which can be a source of contamination, in one embodiment, instead of being closed during their “off” step, gas inlets630and634are kept on for injecting a small amount of H2purge gas. Ideally, the amount of H2purge gas flow is sufficiently small to prevent interference with the flow direction in channel610. For example, in step 1, a small amount of H2purge gas is injected from gas inlet634as indicated by arrow646. Similarly, in step 2, a small amount of H2purge gas is injected from gas inlet630as indicated by arrow648. The existence of small amount of H2gas that flows in the reverse direction of the precursor gases creates turbulence around the gas inlets, thus preventing the precursor gases from depositing Si around the gas inlets.

Returning toFIG. 6C, in addition to gas nozzles612and614, chamber602is also coupled to gas inject nozzles616and618for injecting H2purge gas between the back sides of susceptors604and608and the inner walls of chamber602. The gas pressure between the back sides of susceptors604and608and the inner walls of chamber602is kept equal or more than the gas pressure inside channel610, thus preventing TCS gas contained in channel610to leak into the space next to the inner chamber wall. The existence of an H2gas flow between the back sides of susceptors and the chamber walls further reduces the risk of depositing Si onto the inner walls of chamber602. Similarly, as long as H2can be injected, the positions of gas inject nozzles616and618can be different than the ones shown inFIG. 6C, or the number of H2gas inject nozzles can be fewer or more than two.

FIG. 7illustrates an exemplary multi-chamber reaction module in accordance with one embodiment of the present invention. As illustrated inFIG. 7, a heating lamp is “sandwiched” between two process chambers. Therefore, the radiant energy from the heating lamp can be fully utilized for the deposition.