Semiconductor devices may be fabricated with deep trenches. As used herein, a deep trench is a trench whose bottom is deeper than a metallurgical junction of an implant in the semiconductor, and that may penetrate through most of or an entirety of a layer of the semiconductor device that active devices are fabricated in, such as an epitaxial layer. In devices having the epitaxial layer formed on a substrate, the deep trench may partly penetrate the substrate as well. These trenches may be filled with a dielectric, such as an oxide, or (in low voltage applications) with undoped polysilicon.
One use for deep trenches is in edge termination of the active area of a semiconductor device. These edge terminations may operate to spread the electrostatic potential at the edges of the device, thereby increasing a breakdown voltage of the device. Accordingly, these edge terminations may also be referred to as high-voltage terminations.
A high-voltage termination may include a field plate formed in a metal layer, coupled to an electrode of the semiconductor device, and extending over a portion of a deep trench. To provide a high breakdown voltage (for example, higher than 1200 V) the trench must not only be deep but also must be relatively wide. Typically, a wide trench has a width that is at least half the depth of the trench.
Reliably filling a wide deep trench with a material both capable of supporting such high breakdown voltage and compatible with later processing steps of the semiconductor may be difficult, especially in semiconductor technologies that require high temperatures in processing steps that follow the formation and filling of the wide deep tranches.
Embodiments relate to semiconductor devices and manufacturing processes thereof, and in particular to semiconductor devices including deep trenches and processes for filling such deep trenches.
In embodiments, a semiconductor device comprises a deep trench, a conductive material disposed in the deep trench, and a dielectric disposed within the deep trench and separating the conductive material from surfaces of the deep trench.
In an embodiment, the conductive material is carbon.
In an embodiment, the carbon is formed by pyrolysis of an organic material such as a photoresist.
In an embodiment, the deep trench and the conductive material operate as part of a high-voltage termination of an active device of the semiconductor device.
In embodiments, a method of manufacturing a semiconductor device comprises forming a trench in the semiconductor device, depositing an organic compound within the trench; and converting the organic compound to a carbon fill by converting the organic compound to carbon using pyrolysis.
In an embodiment, the organic compound is thinned before it is converted to carbon.
In an embodiment, the trench includes a layer of dielectric disposed over surfaces of the trench. The layer of dielectric may be formed before the organic compound is deposited.
In an embodiment, a dielectric layer may be formed over the carbon fill.
In an embodiment, the organic compound is a photoresist.
Embodiments of the present application relate to the filling of trenches in semiconductor devices, such as deep trenches used to provide high-voltage terminations. In embodiments, a trench in a semiconductor device is filled with an organic material, such as a photoresist, and then the organic material is converted to carbon by pyrolysis.
A detailed description of embodiments is provided below along with accompanying figures. The scope of this disclosure is limited only by the claims and encompasses numerous alternatives, modifications, and equivalents. Although steps of various processes are presented in a given order, embodiments are not necessarily limited to being performed in the listed order. In some embodiments, certain operations may be performed simultaneously, in an order other than the described order, or not performed at all.
Numerous specific details are set forth in the following description. These details are provided to promote a thorough understanding of the scope of this disclosure by way of specific examples, and embodiments may be practiced according to the claims without some of these specific details. Accordingly, the specific embodiments of this disclosure are illustrative, and are not intended to be exclusive or limiting. For the purpose of clarity, technical material that is known in the technical fields related to this disclosure has not been described in detail so that the disclosure is not unnecessarily obscured.
Power semiconductor devices require a dedicated high-voltage termination to avoid early breakdown at an active area periphery. However, structures used for high-voltage termination come with a significant parasitic capacitance which negatively affects the high-frequency switching performance and, depending on the design and material used, have hysteresis properties, unsuitable for high-frequency operations. To mitigate this issue, deep trench terminations were developed, but at the expense of a higher process complexity to fill the trench.
Super-junction technologies commonly rely on deep trenches etch and semiconductor epitaxial growth fill. The focus of these technologies is to optimize the trade-off between a low on-state resistance and a high blocking voltage, In particular, super-junction devices can break the conventional silicon limit for power semiconductor devices. However, this concept is not suitable for high-voltage terminations and super-junction devices need specific high-voltage termination designs.
Narrow trenches filled with polysilicon are widely used in low-voltage silicon technologies. But this technique is not a viable option to fill the wide trenches required for high-voltage technologies.
Filling wide deep trenches with oxides can be problematic because voids and crevasses may form in the oxide as it is deposited. This makes the characteristics of the deposited oxide somewhat unpredictable and makes the oxide difficult to planarize.
Filling deep trenches with other dielectrics, such as polyimide or benzocyclobutene (BCB), may be impractical because these materials may be unstable at temperatures used in later steps of the manufacturing process: polyimides are unstable above 500° C., and BCB above 350° C., while some steps in the fabrication of high band-gap semiconductors such as silicon carbide (SiC) require temperatures of 700 to 1200° C., or higher.
Polysilicon can be used to fill trenches but is not usable for high-voltage terminations with high breakdown voltages. Trenches for high-voltage SiC technology must be wide to provide an adequate blocking voltage, and a thick polysilicon deposition would be required to fill them. Thick polysilicon films are not practical, in particular due to wear-out of the equipment (tube) in which the deposition of the polysilicon is performed. As a result, typical polysilicon thicknesses are less than 0.6 which is insufficient to fill a wide trench. Furthermore, the high dielectric constant of polysilicon makes it difficult to design polysilicon-filled trenches with a high blocking voltage.
Embodiments include high-voltage terminations for a semiconductor device, the high-voltage termination comprising a deep trench filled with an interfacial dielectric layer and a conductive material to provide a high blocking voltage with a low parasitic capacitance. In embodiments, the conductive material is carbon, and the carbon is formed in the trench by depositing a layer of organic material such as a photoresist and converting the organic material to carbon by pyrolysis.
In an embodiment, the surfaces of the trench are passivated, a thin interfacial dielectric layer (silicon dioxide (SiO2)) is formed on the surfaces of a trench, and then a thick interfacial dielectric layer and a conductive material are deposited to fill the remainder of the trench. The thin interfacial dielectric layer can be either kept (liner oxide) or removed (sacrificial oxide) before the next process step. When used as a high-voltage termination, this structure provides a very efficient field plate effect when the conductive material is electrically grounded (including to a virtual ground), and a blocking voltage above 95% of the epitaxy capability can be achieved. If the conductive layer is electrically floating, the parasitic capacitance of the high-voltage termination is reduced, at the expense of a lower blocking voltage.
Best results are achieved with deep trenches; ideally through the whole epitaxy layer, but at least deeper than a metallurgical junction of a doped structure (such as a p-well) of the device. A large trench width is also beneficial for the blocking voltage and well-suited to the manufacturing process of the embodiments.
To overcome the difficulties with filling a deep trench, manufacturing processes according to embodiments are based on the deposition of a photoresist and subsequent conversion of the photoresist to carbon by pyrolysis (heat). A photoresist deposition provides an excellent filling (no holes or cracks) and is easily planarized by spinning. The etch of the photoresist is also a standard process with excellent uniformity
Embodiments are particularly well-suited for SiC technologies (which may have a thin epitaxy layer and high-voltage rating) but is also applicable to a wide range of other semiconductor materials, including but not limited to silicon and gallium nitride (GaN). The pyrolysis-formed carbon provides a conductive element that fills the entire trench and conforms well to the imperfections of the thick interfacial dielectric layer of the trench, preventing the formation of mounds, bumps, and valleys. Furthermore, during the pyrolysis process, the photoresist will induce less stress than trench-filling technologies of the related art.
The device 100 comprises a substrate 102 on which an epitaxial layer (hereinafter, the epitaxy 104) has been formed. In embodiments, the substrate 102 and epitaxy may be a wide bandgap semiconductor such as SiC, and may be n-type material, but embodiments are not limited thereto.
A doped region 106 is formed in the epitaxy 104 between two deep trenches 108. In the illustrated PIN diode, the doped region 106 is a p-type region, but embodiments are not limited thereto.
Each of the trenches 108 is lined with an dielectric 118 that is also formed over the top of the epitaxy 104 and the doped region 106. In embodiments, the dielectric 118 includes silicon dioxide.
A carbon fill 116 is formed inside the dielectric 118 lining each trench 108. In embodiments, the carbon fill 116 is formed by pyrolysis of an organic material, such as a photoresist. The carbon fill 116 in the embodiment of
A first electrode 126 is formed over and in electrical contact with the doped region 106 and extends over at least part of carbon fill 116 in the trenches 108. The first electrode 126 provides an electrical connection to an anode of the vertical PIN diode comprised of the doped region 106, the portion of the epitaxy 104 below the doped region 106, and the substrate 102.
The first electrode 126 may comprise aluminum, among other conductors, and may be electrically connected to the doped region 106 through a thin metal layer 122, which in an embodiment may comprise nickel silicide.
The portion of the first electrode 126 extending over the carbon fill 116 in the trenches 108 may operate as a field plate to increase the breakdown voltage of the PIN diode of device 100.
A second electrode 130 comprising a conductor, such as silver, is formed over a bottom surface of the substrate 102 and provides an electrical connection to a cathode of the PIN diode.
A passivation layer 128 is formed over the electrode 126 and the dielectric 108. In embodiments, the passivation layer comprises silicon oxynitride (SiON).
the trenches 108S of
the carbon fill 116S of
The trenches 108S are still deep trenches because they are still deeper than the metallurgical junction between the bottom of the doped region 106 and the epitaxy 104.
Compared to the floating carbon fill 116 of
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Forming a thin oxide layer 410 passivates the surface of the trench 408, which can improve, among other characteristics, the leakage current and the breakdown voltage of the device. However, forming the thin oxide layer 410 is optional.
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The device 1100 comprises a substrate 1102 on which an epitaxy 1104 has been formed.
Doped regions 1106 are formed in active areas 1112A and 1112B (collectively, active regions 112) of the epitaxy 1104 between deep trenches 1108. The doped regions 1106 correspond to body regions of an n-channel VMOSFETs, may be p-type regions, and accordingly may include a heavily doped n-type source regions 1126 formed towards a center of the respective active region 1112 and a heavily doped p-type region 1128 formed towards a peripheral region of that active region 1112 and adjacent to an n-doped source region 1126.
Each of the trenches 1108 is lined with a dielectric 1136 that is also formed over the top of the epitaxy 1104 and the doped region 1106 and that may form a gate dielectric under a gate electrode 1138. In embodiments, the dielectric 1136 includes silicon dioxide.
A carbon fill 1116 is formed inside the dielectric 1136 lining each trench 1108. In embodiments, the carbon fill 1116 is formed by pyrolysis of an organic material, such as a photoresist. The carbon fill 1116 in the embodiment of
First electrodes 1142 are formed over and in electrical contact with the doped regions 1126 and 1128, and extend over at least parts of carbon fill 1116 in the trenches 1108. The first electrodes 1142 provides electrical connections to source and body regions of the VMOSFETs of device 1100.
The first electrode 1142 may comprise aluminum, among other conductors, and is electrically connected to the doped regions 1126 and 1128. In some embodiments, the first electrode 1142 is electrically connected to the doped regions 1126 and 1128 through a thin conductive layer (not shown).
The portion of the first electrode 1142 extending over the carbon fill 1116 in the trenches 1108 may operate as a field plate to increase the breakdown voltage of the VMOSFET of device 1100.
A second electrode 1160 comprising a conductor, such as silver, is formed over a bottom surface of the substrate 1102 and provides an electrical connection to drains of the VMOSFETs in the active areas 1112.
A passivation layer 1144 is formed over the first electrode 1142 and the dielectric 1136. In embodiments, the passivation layer comprises silicon oxynitride (SiON).
In the device 1300 shown in
In another embodiment, two trenches replace the single trench 1108 shown in the middle of
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Illustrative embodiments have been provided wherein deep trenches are lined with a dielectric, such as silicon dioxide, and then filled with pyrolytically-formed carbon. Such deep trenches may be used to provide high-voltage termination to semiconductor devices, and may improve the breakdown voltages of those devices, decrease a junction capacitance of those devices, or a combination thereof. Because it is easier to produce a deep trench filled according to embodiments than it is to produce a deep trench filled with oxide only, manufacturing of high-frequency and high-voltage semiconductor devices is made easier.
Aspects of the present disclosure have been described in conjunction with the specific embodiments that are presented as illustrative examples, but embodiments are not limited to those shown in the drawings or those mentioned in the accompanying text. Numerous alternatives, modifications, and variations to the disclosed embodiments may be made without departing from the scope of the claims set forth below. Embodiments disclosed herein are not intended to be limiting.