Patent Application
20130012030
1. Field of the Invention
Embodiments of the invention relate to an apparatus and method for forming solar cells. More particularly, embodiments of the present invention relate to an apparatus and method for forming amorphous and microcrystalline silicon layers utilized in solar cell applications.
2. Description of the Related Art
Photovoltaic (PV) devices or solar cells are devices which convert sunlight into direct current (DC) electrical power. Typical thin film PV devices, or thin film solar cells, have one or more p-i-n junctions. Each p-i-n junction comprises a p-type layer, an intrinsic type layer, and an n-type layer. When the p-i-n junction of the solar cell is exposed to sunlight (consisting of energy from photons), the sunlight is converted to electricity through the PV effect. Solar cells may be tiled into larger solar arrays.
Typically, a thin film solar cell includes active regions, or photoelectric conversion units, and a transparent conductive oxide (TCO) film disposed as a front electrode and/or as a back electrode. The photoelectric conversion unit includes a p-type silicon layer, an n-type silicon layer, and an intrinsic type (i-type) silicon layer sandwiched between the p-type and n-type silicon layers. Several types of silicon films including microcrystalline silicon film (μc-Si), amorphous silicon film (a-Si), polycrystalline silicon film (poly-Si), and the like may be utilized to form the p-type, n-type, and/or i-type layers of the photoelectric conversion unit. The backside electrode may contain one or more conductive layers.
Both amorphous and microcrystalline silicon films are currently being used to form solar cells. However, problems exist in current production equipment and methods used in the deposition of these films. For example, in conventional thermal chemical vapor deposition and plasma enhanced chemical vapor deposition (PECVD) processes, the low energy gas phase combination of silicon and hydrogen leads to the formation of polymerized silicon and hydrogen structures, which can lead to particle generation, inefficient film deposition, and physically and electrically inferior and unstable deposited films.
Therefore, there is a need for an improved apparatus and method for depositing amorphous and microcrystalline silicon films.
In one embodiment of the present invention, a method for depositing a silicon-containing film comprises generating hydrogen radicals remotely from a processing chamber, introducing a flow of the hydrogen radicals into a processing region of the processing chamber, wherein a substrate is positioned in the processing region, introducing a flow of silicon-containing gas into the processing region of the processing chamber, and depositing the silicon film on the substrate. The remotely generated hydrogen radicals are not mixed with the silicon-containing gas prior to reaching the processing region.
In another embodiment, a method for depositing a silicon-containing film comprises establishing a flow of argon gas into a remote plasma source, igniting a plasma within the remote plasma source, establishing a flow of hydrogen gas into the remote plasma source such that a flow of hydrogen radicals is established, delivering the flow of hydrogen radicals into a processing region of a processing chamber, wherein a substrate is positioned in the processing region, generating a flow of silicon-containing gas into the processing region of the processing chamber, and depositing the silicon film on the substrate. The hydrogen radicals are not mixed with the silicon-containing gas prior to reaching the processing region of the processing chamber.
In yet another embodiment of the present invention, an apparatus for depositing a silicon-containing film comprises a processing chamber having a plurality of walls, a showerhead, and a substrate support that define a processing region within the processing chamber, a silicon-containing gas source coupled to the processing region through a first plurality of gas passages disposed through the showerhead, a remote plasma source coupled to a hydrogen gas source and configured to generate a plurality of hydrogen radicals therein, line of sight tubing coupling the remote plasma source to the processing chamber, wherein the line of sight tubing comprises an inert material, and a feed tube coupling the line of sight tubing to the processing region such that hydrogen radicals delivered by the feed tube do not mix with a silicon-containing gas prior to entering the processing region.
So that the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.
Embodiments of the present invention generally provide improved apparatus and methods for depositing amorphous and microcrystalline silicon films during the formation of solar cells. In one embodiment, a method and apparatus is provided for generating and introducing hydrogen radicals directly into a processing region of a processing chamber for reaction with a silicon-containing precursor for film deposition on a substrate. In one embodiment, the hydrogen radicals are generated by a remote plasma source and directly introduced into the processing region via a line of sight path to minimize the loss of energy by the hydrogen radicals prior to reaching the processing region. The line of sight path may include tubing formed from a non-reactive material, such as a dielectric or ceramic material. In some configurations, it is desirable to heat the tubing to reduce the possible transfer of energy to the tubing and prevent adsorption of the hydrogen radicals onto the surface of the tubing prior to introduction into the processing region.
In one configuration, the first p-i-n junction 120 may comprise a p-type amorphous silicon layer 122, an intrinsic type amorphous silicon layer 124 formed over the p-type amorphous silicon layer 122, and an n-type amorphous silicon layer 126 formed over the intrinsic type amorphous silicon layer 124. In one example, the p-type amorphous silicon layer 122 may be formed to a thickness between about 60 Å and about 300 Å, the intrinsic type amorphous silicon layer 124 may be formed to a thickness between about 1,500 Å and about 3,500 Å, and the n-type amorphous semiconductor layer 126 may be formed to a thickness between about 100 Å and about 500 Å. The back contact layer 150 may include, but is not limited to, aluminum (Al), silver (Ag), titanium (Ti), chromium (Cr), gold (Au), copper (Cu), platinum (Pt), alloys thereof, or combinations thereof.
The first p-i-n junction 220 may comprise a p-type amorphous silicon layer 222, an intrinsic type amorphous silicon layer 224 formed over the p-type amorphous silicon layer 222, and an n-type microcrystalline silicon layer 226 formed over the intrinsic type amorphous silicon layer 224. In one example, the p-type amorphous silicon layer 222 may be formed to a thickness between about 60 Å and about 300 Å, the intrinsic type amorphous silicon layer 224 may be formed to a thickness between about 1,500 Å and about 3,500 Å, and the n-type microcrystalline semiconductor layer 226 may be formed to a thickness between about 100 Å and about 400 Å.
The second p-i-n junction 230 may comprise a p-type microcrystalline silicon layer 232, an intrinsic type microcrystalline silicon layer 234 formed over the p-type microcrystalline silicon layer 232, and an n-type amorphous silicon layer 236 formed over the intrinsic type microcrystalline silicon layer 234. In one embodiment, prior to deposition of the intrinsic type microcrystalline silicon layer 234, an intrinsic microcrystalline silicon seed layer 233 may be formed over the p-type microcrystalline silicon layer 232. In one example, the p-type microcrystalline silicon layer 232 may be formed to a thickness between about 100 Å and about 400 Å, the intrinsic type microcrystalline silicon layer 234 may be formed to a thickness between about 10,000 Å and about 30,000 Å, and the n-type amorphous silicon layer 236 may be formed to a thickness between about 100 Å and about 500 Å. In one embodiment, the intrinsic microcrystalline silicon seed layer 233 may be formed to a thickness between about 50 Å and about 500 Å. The back contact layer 250 may include, but is not limited to, aluminum (Al), silver (Ag), titanium (Ti), chromium (Cr), gold (Au), copper (Cu), platinum (Pt), alloys thereof, or combinations thereof.
Current methods of depositing the various amorphous and microcrystalline silicon films to form the solar cell 100, 200 include introducing a mixture of hydrogen-based gas, such as hydrogen gas (H2), and silicon-based gas, such as silane (SiH4), into a processing region of a plasma enhanced chemical vapor deposition (PECVD) processing chamber, exciting the gas mixture into a plasma, and depositing the desired film on the substrate 102. During this process, two types of bonds are formed and deposited onto the substrate, namely Si—H bonds and Si—H2 bonds. It has been found that the H2 bonds are undesirable because they form particles or defects in the deposited film, resulting in less efficient, lower quality bonds and film deposition. Therefore, it is desirable to increase Si—H bond formation and reduce Si—H2 bond formation during the deposition process. Additionally, it is desirable to reduce polymerization of silicon into long chain polymers, which also results in defects formed in and instability of the deposited films. Embodiments of the present invention accomplish these results by directly introducing hydrogen radicals into the processing region of the processing chamber separately from the silicon-based gas, such that the hydrogen radicals combine with the silicon-based gas to produce significantly more Si—H bonds during the deposition process than current methods and apparatus. It is believed that the use of conventional plasma processing techniques, which use a single capacitively or inductively coupled plasma source to deliver energy to a combination of processing gases (e.g., silane and hydrogen gas) disposed in a processing region of a processing chamber, are not effective or efficient in coupling the RF power to the hydrogen atoms in the process gas mixture to create a desirable percentage of reactive hydrogen radicals to form the more desirable Si—H bonds versus the Si—H2 bonds in the deposited silicon layer. In one example, it is believed that a single capacitively coupled plasma source, such as a RF driven showerhead disposed over a substrate, is only able to convert about 10-20% of hydrogen atoms in a silane and hydrogen gas mixture into hydrogen radicals. Therefore, by use of the combination of a capacitively or inductively coupled plasma source that delivers energy to a process gas mixture comprising hydrogen radicals delivered from a remote plasma source and a silicon-containing gas delivered from a separate gas source, the deposited film quality and electrical characteristics of the deposited film can be greatly improved. For instance, embodiments of the present invention yield hydrogen radical delivery to the process chamber on the order of 30-70% as opposed to the prior art 10-20%. It should be noted that the term “hydrogen radical” as used herein denotes a single, highly reactive, neutral hydrogen atom.
The showerhead 310 is coupled to a backing plate 312 at its periphery by a suspension 314. The showerhead 310 may also be coupled to the backing plate by one or more center supports 316 to help prevent sag and/or control the straightness/curvature of the showerhead 310. A gas source 320 is configured to supply a processing gas, such as a silicon-containing gas, through a gas feed tube 345. In one embodiment, the gas feed tube 345 is an annular tube configured to feed the processing gas to the processing region 306 through a plurality of gas passages 311 in the showerhead 310.
A hydrogen gas source 390 is fluidly coupled to a remote plasma source 324, such as an inductively coupled remote plasma source. The remote plasma source 324 is also fluidly coupled to the processing region 306 through line of sight tubing 347 and a central feed tube 349. The line of sight tubing 347 fluidly couples the remote plasma source 324 to the central feed tube 349. The term “line of sight” used herein is meant to convey a short distance between the remote plasma source 324 and the processing chamber 300 so as to minimize the possibility of hydrogen radical recombination or adsorption onto the surface of the tubing. In one embodiment, the line of sight tubing 347 provides a direct path for the hydrogen radicals without any sharp bends therein. In one embodiment, the line of sight tubing 347 provides a direct path for the hydrogen radicals without any bends therein. The line of sight tubing 347 comprises tubing made of an inert material, such as sapphire, quartz, or other ceramic material, to prevent adsorption and/or recombination of the hydrogen radicals provided by the remote plasma source 324. Additionally, a heater jacket 351 may be provided to further prevent adsorption and/or recombination of the hydrogen radicals provided by the remote plasma source 324 prior to their delivery into the processing region 306. The line of sight tubing 347 and the central feed tube 349 are configured to provide a direct, short path for hydrogen radicals generated in the remote plasma source 324 into the processing region 306. In one embodiment, the central feed tube 349 is configured to directly feed hydrogen radicals generated in the remote plasma source 324 through a central opening 353 in the showerhead 310 into the processing region 306, as shown in
In one embodiment, the processing chamber 300 also includes a cleaning gas remote plasma source 395 that is fluidly coupled to a gas plenum 397, located behind the showerhead 310, and further coupled to the processing region 306 through the gas passages 311 formed in the showerhead 310. The cleaning gas remote plasma source 395 is coupled to a cleaning gas source 396 that is able to deliver a cleaning gas to the cleaning gas remote plasma source 395 so that energetic cleaning gases can be formed to clean the surfaces of the showerhead 310 and other chamber components between deposition processes. Typical cleaning gases include halogen-containing gases, such as NF3, F2, Cl2, or other gases which are used to remove portions of deposited material formed on chamber components during prior deposition processes. One will note that while the positioning of an outlet 398 of the cleaning gas remote plasma source 395, as illustrated in
An RF power source 322 is coupled to the backing plate 312 and/or to the showerhead 310, 410 to provide a RF power to the showerhead 310, 410 so that an electric field is created between the showerhead 310, 410 and the substrate support 330 or chamber walls 302. Thus, a capacitvely coupled plasma is generated in the processing region 306 for depositing a film on the substrate 102. A vacuum pump 309 is also coupled to the processing chamber 300 through a throttle valve 380 to control the processing region 306 at a desired pressure.
Regardless of the specific embodiment, the gas source 320, remote plasma source 324, and the showerhead 310, 410 are configured such that hydrogen radicals generated in the remote plasma source 324 are introduced to the processing gas only within the processing region 306 in order to prevent undesirable mixing and undesirable deposition in other regions of the processing chamber 300. Further, the hydrogen radicals are delivered directly into the processing region 306 to minimize recombination or energy loss by the hydrogen atoms prior to mixing with the processing gas(es) disposed in the processing region 306. Thus, undesirable the undesirable Si—H2 bonds are minimized and the desirable Si—H bonds are maximized to provide better more efficient silicon film deposition.
In one embodiment, hydrogen radicals are generated within one or more remote plasma sources, such as the remote plasma source 324 depicted in
In one embodiment, it is desirable to adjust the pressure, gas flow rates, and/or ratio of gases, such as carrier gases (e.g., argon) to hydrogen ratio, delivered to the plasma generation region in the remote plasma source 324 to prevent the plasma generated therein from extinguishing, when the composition and/or pressure in the processing region 306 of the processing chamber 300 is varied during the deposition processes performed on the substrate 102.
An example of the deposition methods used to form the amorphous and microcrystalline silicon layers contained in the solar cells 100 and 200 of
In one embodiment, the heating and/or cooling elements 339 are set to provide a substrate support temperature during deposition of about 400 degrees Celsius or less, preferably between about 150 degrees Celsius and about 400 degrees Celsius. The spacing during deposition between the top surface of the substrate 102 disposed on the substrate receiving surface 332 and the showerhead 310, 410 may be between about 200 mil and about 1,000 mil.
For deposition of the silicon films, a silicon-based gas is generally provided by the gas source 320. Suitable silicon based gases include, but are not limited to silane (SiH4), disilane (Si2H6), silicon tetrafluoride (SiF4), silicon tetrachloride (SiCl4), dichlorosilane (SiH2Cl2), and combinations thereof. The p-type dopants of the p-type layers may each comprise a group III element, such as boron or aluminum. Examples of boron-containing sources include trimethylboron (TMB), diborane (B2H6), and similar compounds. The n-type dopants of the n-type silicon layers may each comprise a group V element, such as phosphorus, arsenic, or antimony. Examples of phosphorus-containing sources include phosphine and similar compounds. The dopants are typically provided with a carrier gas, such as hydrogen, argon, helium, and other suitable compounds.
The following illustrates an example of a processing sequence that may be used to form a tandem cell, such as the solar cell 200 illustrated in
Next, the substrate 102 may be transferred into another processing chamber, which is similarly configured to the processing chamber 300, for deposition of an intrinsic type amorphous silicon layer 124 over the p-type amorphous silicon layer 122. In one embodiment, silane gas is provided at a flow rate between about 0.5 sccm/L and about 7 sccm/L from the gas source 320, through the gas feed tube 345, and through the plurality of gas passages 311 in the showerhead 310, 410 into the processing region 306. Simultaneously, hydrogen radicals, generated in the remote plasma source 324 according to the description provided above with respect to
Next, while the substrate 102 is still in the processing chamber 300, an n-type microcrystalline silicon layer 126 is deposited on the intrinsic type amorphous silicon layer 124. In one embodiment, silane gas is provided at a flow rate between about 0.1 sccm/L and about 0.8 sccm/L, such as about 0.35 sccm/L from the gas source 320, through the gas feed tube 345, and through the plurality of gas passages 311 in the showerhead 310, 410 into the processing region 306. Simultaneously, hydrogen radicals, generated in the remote plasma source 324 according to the description provided above with respect to
Next, the substrate 102 is moved to another processing chamber 300 for depositing a p-type microcrystalline silicon layer 132 over the n-type microcrystalline silicon layer 126. In one embodiment, silane gas is provided at a flow rate between about 0.1 sccm/L and about 0.8 sccm/L from the gas source 320, through the gas feed tube 345, and through the plurality of gas passages 311 in the showerhead 310, 410 into the processing region 306. Simultaneously, hydrogen radicals, generated in the remote plasma source 324 according to the description provided above with respect to
Next, the substrate 102 is transferred into another processing chamber 300 for deposition of the intrinsic type microcrystalline silicon seed layer 133 over the p-type microcrystalline silicon layer 132. In one embodiment, silane gas is gradually ramped up from a zero point to a second set point, such as between about 2.8 sccm/L and about 5.6 sccm/L over a time period from about 20 seconds to about 300 seconds, such as between about 40 seconds and about 240 seconds. The ramped up silane flow is provided from the gas source 320, through the gas feed tube 345, and through the plurality of gas passages 311 in the showerhead 310, 410 into the processing region 306. Simultaneously, hydrogen radicals, generated in the remote plasma source 324 according to the description provided above with respect to
It is believed that the gradual ramp-up of the silane gas flow in the intrinsic type microcrystalline silicon seed layer 133 formation assists silicon atoms in uniformly adhering and distributing on the surface of the substrate 102, thereby forming the intrinsic type microcrystalline silicon seed layer 133 with desirable film properties. Uniform adherence of the silicon atoms on the surface of the substrate 102 provides good nucleation sites for subsequent atoms to nucleate thereon. Uniform nucleation sites formed on the substrate 102 promote crystallinity of films subsequently formed thereon. Therefore, the gradual ramp-up of the silane flow into the processing region 306 allows the dissociated silicon atoms to have sufficient time to be gradually absorbed on the surface of the substrate 102, thereby providing a surface having an even distribution of silicon atoms that provides nucleation sites, which promote improved crystallinity of subsequently deposited layers.
Next, an intrinsic type microcrystalline silicon layer 134 is deposited over the intrinsic type microcrystalline silicon seed layer 133 in the processing chamber 300. Silane gas may be provided at a flow rate between about 0.5 sccm/L and about 5 sccm/L from the gas source 320, through the gas feed tube 345, and through the plurality of gas passages 311 in the showerhead 310, 410 into the processing region 306. Simultaneously, hydrogen radicals, generated in the remote plasma source 324 according to the description provided above with respect to
Finally, while the substrate is still positioned in the processing chamber 300, an n-type amorphous silicon layer 126 is deposited over the intrinsic type microcrystalline silicon layer 124 on the substrate 201. In one embodiment, the n-type amorphous silicon layer 136 may be deposited by first depositing an optional first n-type amorphous silicon layer at a first silane flow rate and then depositing a second n-type amorphous silicon layer over the first optional n-type amorphous silicon layer at a second silane flow rate lower than the first silane flow rate. The first optional n-type amorphous silicon layer may be deposited by providing silane gas at a flow rate between about 1 sccm/L and about 10 sccm/L, such as about 5.5 sccm/L from the gas source 320, through the gas feed tube 345, and through the plurality of gas passages 311 in the showerhead 310, 410 into the processing region 306. Simultaneously, hydrogen radicals, generated in the remote plasma source 324 according to the description provided above with respect to
The second n-type amorphous silicon layer deposition may comprise providing silane gas at a flow rate between about 0.1 sccm/L and about 5 sccm/L, such as about 0.5 sccm/L and about 3 sccm/L, for example about 1.42 sccm/L from the gas source 320, through the gas feed tube 345, and through the plurality of gas passages 311 in the showerhead 310, 410 into the processing region 306. Simultaneously, hydrogen radicals, generated in the remote plasma source 324 according to the description provided above with respect to
Thus, each of the silicon-containing layers in a solar cell may be provided by generating hydrogen radicals in a remote plasma source and delivering the hydrogen radicals directly into the processing region of the processing chamber to combine with the silicon-containing gas according to embodiments of the present invention. Directly providing the hydrogen radicals into the processing region for reaction with the silicon-containing gas results in improved bonding structure, deposition efficiency, and deposited film stability over prior art deposition methods.
While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CN2010/000325 | 3/17/2010 | WO | 00 | 9/12/2012 |
