1. Field
The present disclosure relates generally to high-voltage field effect transistors (FETs), and, more specifically, the present disclosure relates to improved fabrication processes for manufacturing high-voltage FETs.
2. Background
Many electrical devices such as cell phones, personal digital assistants (PDAs), laptops, etc. utilize power to operate. Because power is generally delivered through a wall socket as high voltage alternating current (AC), a device, typically referred to as a power converter can be utilized to transform the high-voltage AC input to a well regulated direct current (DC) output through an energy transfer element. Switched mode power converters are commonly used to improve efficiency, size, and reduce component count in many of today's electronics. A switch mode power converter may use a power switch that switches between a closed position (ON state) and an open position (OFF state) to transfer energy from an input to an output of the power converter. Typically, power switches are high-voltage devices required to withstand voltages substantially greater than the AC input voltage.
One type of high-voltage FET is the heterostructure FET (HFET), also referred to as a high-electron mobility transistor (HEMT). HFETs may be used as switches in switching devices for high-voltage power electronics, such as power converters. In certain applications, HFETs based on wide bandgap semiconductors may be useful because the higher bandgap may improve performance at elevated temperatures. Examples of wide bandgap semiconductors used in high-voltage HFETs include materials such as silicon carbide (SiC), gallium nitride (GaN), and diamond, although other materials may be used as well.
Various aspects, features, and advantages of several embodiments of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings.
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following Figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention.
Reference throughout this specification to “one embodiment,” “an embodiment,” “one example,” or “an example” means that a particular feature, structure or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” “one example,” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable combinations and/or subcombinations in one or more embodiments or examples. Particular features, structures, or characteristics may be included in an integrated circuit, an electronic circuit, a combinational logic circuit, or other suitable components that provide the described functionality. In addition, it is appreciated that the Figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.
In the description below, an example FET is used for the purposes of explanation. The example FET is referred to as an HFET despite the FET having a gate dielectric. In this respect, the example FET could also be called a metal insulator semiconductor FET (MISFET). For ease of explanation, however, the term HFET is used. It should be understood that use of this term is not limiting on the claims.
As shown, a gate electrode 118 above a gate dielectric layer 108 controls the current conduction path between source electrode 112 and drain electrode 114. Gate dielectric layer 108 may also act as a passivation layer for the surface of AlGaN film 106. Specifically, gate dielectric layer 108 functions as a passivation layer for the “un-gated” region (i.e., the region between the edge of the gate and the source and the region between the other edge of the gate and the drain). Gate field plate (GFP) 116, formed on top of field plate dielectric 110, may alleviate the electric field intensity at an edge (closest to the drain) of gate electrode 118 and may also reduce leakage current by controlling the states of charge traps at the interface between AlGaN film 106 and gate dielectric layer 108.
In addition to leakage current, examples of other potential concerns for HFET design are current collapse and gate dielectric breakdown. Current collapse, which is the unintended reduction of drain current during operation or in a stressed state, may be caused by charge trapping at the surface of the AlGaN film or elsewhere in the AlGaN and GaN layers. In addition to improving the passivation of the surface of the AlGaN film, a field plate may also be used to reduce current collapse.
Gate dielectric breakdown is the electrical shorting of the gate electrode to the AlGaN/GaN films and results from a dfective or over-stressed gate dielectric. Higher quality gate dielectrics may improve an HFET's breakdown performance and long-term reliability.
In block 202, as illustrated in
AlGaN film 302, which is used as a barrier film, may be 10 to 40 nm thick. GaN film 304, which forms the channel film, may be about 0.3 to 5 μm thick. However, other thicknesses may also be used.
In block 204, as illustrated in
Referring to
In one example, first film 402 may also function as a passivation layer that forms a high quality interface with AlGaN film 302. The quality of the passivation layer may affect the quality of the interface with AlGaN film 302, which may affect the onset of current collapse due to carrier traps and trapped charge at the interface.
A second film 404 deposited in the ALD tool may be a dielectric material that is deposited directly onto first film 402 and serves as a field plate dielectric film for separating a field plate from the gate dielectric and AlGaN film 302. In some cases, the material of the second film may be selected to have a specific electrical property. For example, the material for second film 404 may depend on properties such as permittivity, refractive index, defect density, stability, and mechanical stress. In other cases, the material of the second film may be selected for integration or fabrication reasons. For example, the material for second film 404 may depend on the material and thickness of first film 402 so that second film 404 may be properly etched while maintaining the integrity of first film 402. In one example, second film 404 may be selected to have sufficiently different etch properties from first film 402 so that first film 402 may be used as an etch-stop when etching second film 404. Specifically, second film 404 may be chosen so that the etch selectivity ratio between first film 402 and second film 404 may be at least 5. Etch selectivity is the ratio of etch rate of a first material to the etch rate of a second material. For example, an etch selectivity of 10 to a first material over a second material means that the amount (e.g., thickness) of etched first material is about 10 times greater than the amount (e.g., thickness) of etched second material. Importantly, while being used as an etch-stop layer, first film 402 should not be completely removed to expose the active semiconductor layer below. Rather, the amount of additional etching into first film 402 that may occur when etching the second film may be minimized to maintain a high-quality gate dielectric (i.e., first film 404). In one example, the thickness of second film 404 may be 80 nm to 200 nm and may be made from, for example, silicon nitride (SiN), Al2O3, silicon dioxide (SiO2), or other suitable materials.
Using ALD techniques, a film thickness may be deposited one layer at a time on a substrate surface, each layer being a fraction of the total film thickness. In one example, each cycle of deposition may be no thicker than one atomic layer due to the self-limiting ALD process. In this example, each cycle of deposition is typically less than one atomic layer because complete coverage is never obtained with each cycle. Deposition of the full desired film thickness is carried out over many cycles of the same sequence of steps. For example, one cycle may include the sequences of: (1) a chemi-sorbtion or chemical dose step, (2) a chemical dose purge, (3) a plasma step, and (4) a post plasma purge. The chemical dose step deposits a thin layer of chemical over a substrate surface. The chemical may, for example, be a precursor necessary to create a layer of the desired film material. A purge step is then performed to remove any remnants of the chemical in the chamber. Next, a plasma step may cause a gas plasma to react with the chemical precursor on the substrate surface, or may include multiple gas plasmas that react with each other on a substrate surface, to create a thin layer of the desired material on the substrate surface. Finally, another purge step is performed to remove remnants of the plasma gas from the chamber. This cycle of steps may be repeated as many times as necessary to obtain the desired thickness of film. One example of an ALD tool for carrying out this type of process is an Oxford ALD FlexAl System. While an example ALD cycle has been explained above with respect to four steps, other ALD cycles with additional, fewer, or different steps may be contemplated.
As explained in more detail below with respect to
In this example, the first and second films are in situ deposited on the wafer in the ALD tool, i.e., without exposing the wafer to the environment outside the tool between the deposition of the two films. Because the films are deposited sequentially without removal from the tool, the wafer is protected from contamination that may degrade the quality of the films or the AlGaN surface. In addition, because the wafer is not removed between depositions of films, the processing throughput of the tool may be increased. In particular, by not removing the wafers between depositions, wafer handling times (e.g., pumping the load lock down to the appropriate vacuum levels, moving wafers from tool to tool, etc.) may be decreased. Thus, the deposition capabilities of the ALD tool may be utilized at a higher rate.
Other potential benefits of using an ALD tool include low temperature processing, moving plasma away from the surface of the wafer (i.e., “remote plasma”), which may help maintain the integrity of the wafer surface, creating ultra high quality films, and depositing in high-aspect ratio holes.
In one example of an ALD process recipe, an Al2O3 film may be deposited in an ALD chamber at 300° C. with a growth rate of about 1.4 A/cycle. Each cycle of the deposition starts with a chemical dose of about 20 ms at 15 mT. In one case, trimethylaluminum (TMA) is used. Next the chamber is purged with 50 cc of nitrogen (N2) and 100 cc argon (Ar) for 1.5 s, which removes the residue of the chemical vapor from the chamber. Next a 2 s 50 cc oxygen (O2) plasma dose step is performed with a plasma power of 400 W. Next, the chamber is purged again for is with 50 cc of N2 and 100 cc Ar to remove the residue of the gas plasma from the chamber. The cycle may then be repeated as many times as necessary to obtain the desired film thickness. For example, a total of 100 cycles would produce a film of approximately 150 A.
In another example of an ALD process recipe, an HfO2 film may be deposited in an ALD chamber at 300° C. with a growth rate of about 1.2 A/cycle. Each cycle of the deposition starts with a chemical dose of about 1.1 s at 80 mT. In one case, tetrakis-(ethylmethylamino)-hafnium (TEMAH) is used with a 200 cc Ar flow through a bubbler. Next the chamber is purged with 100 cc of N2 and 250 cc Ar for 13 s, which removes the residue of the chemical vapor from the chamber. Next a 4 s 50 cc O2 dose plasma treatment step is performed with a plasma power of 250 W at 15 mT. Then the chamber is purged again for 2 s with 100 cc of N2 and 250 cc Ar at 80 mT to remove the residue of the gas plasma from the chamber. The cycle may then be repeated as many times as necessary to obtain the desired film thickness. For example, a total of 17 cycles would produce a film of approximately 20 A.
In yet another example of an ALD process recipe, an AlN film may be deposited in an ALD chamber at 300° C. with a growth rate of about 0.7 A/cycle. Each cycle of the deposition starts with a chemical dose of about 30 ms at 15 mT. In one case, TMA is used. Next the chamber is purged with 100 cc of N2 and 100 cc Ar for 2 s, which removes the residue of the chemical vapor from the chamber. Next a 15 s 30 cc N2 plasma treatment step is performed with a plasma power of 400 W at 10 mT. Then the chamber is purged again for 3 s with 100 cc of N2 and 100 cc Ar at 15 mT to remove the residue of the gas plasma from the chamber. The cycle may then be repeated as many times as necessary to obtain the desired film thickness. For example, a total of 29 cycles would produce a film of approximately 20 A.
These procedures are examples and other variations may be developed that do not take away from the spirit of the invention. For example, an SiN recipe may be developed using similar steps as above with tris[dimethylamino]silane (3DMAS) or bis[tertiary-butylamino]silane (BTBAS) gas being used for the chemical dose and N2, H2, or NH3 for the plasma gas.
In block 206, as illustrated in
In block 208, a rapid thermal anneal (RTA) step is performed, for example, to ensure an ohmic contact between the source and drain electrodes and the AlGaN or GaN films. In one example, the temperature range for an RTA process may be between 500° C. to 850° C. depending on the specific metal stack, surface pre-treatment, and whether the electrodes are formed in a recessed hole that reaches the GaN film. The temperature ramp rate may be about 10 to 15° C./min, and the soak time at the peak temperature may be about 30 s to 1 min.
In block 210, as illustrated in
The sloped side walls of eyelet 602 may reduce the peak electrical field at the edge of the gate electrode towards the drain side. Minimizing the electrical field density along certain surfaces of the gate electrode may increase breakdown voltage and reduce current collapse by preventing the injection of hot carriers into the gate dielectric and passivation layer (i.e., first film 402). However, as described below with respect to
In block 212, as illustrated in
While the example process of flow chart 200 (
As shown in
Adding third film 808 as an etch top layer allows for the first and second films to be made of materials with similar etch properties or be made of the same material. For example, in this example process, the same material may be used for both the first and second films because the third film may protect first film 806 when etching second film 810. In one case, first film 806 may be 150 A of Al2O3, the third film may be 20 A of HfO2, and second film 810 may be 1500 A of Al2O3. In another case, first film 806 may be 150 A of Al2O3, the third film may be 20 A of AlN, and second film 810 may be 1500 A of SiN. Other processes may use different materials and thicknesses.
While optional features, such as the third film, the field plate, and sloped sidewalls of the gate electrode, have been described above with respect to specific HFETs and processes, it should be understand that these features can be mixed and matched in any combination.
The above description of illustrated examples of the present invention, including what is described in the Abstract, are not intended to be exhaustive or to be limitations to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible without departing from the broader spirit and scope of the present invention. Indeed, it is appreciated that the specific examples of thicknesses, materials, processing operations, etc., are provided for explanation purposes, and that other thicknesses, materials, processing operations, etc. may also be employed in other embodiments, examples, and processes in accordance with the teachings of the present invention.
These modifications can be made to examples of the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation. The present specification and Figures are accordingly to be regarded as illustrative rather than restrictive.
Number | Date | Country | |
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Parent | 13323672 | Dec 2011 | US |
Child | 13963923 | US |