Embodiments described herein relate to semiconductor devices having a laterally varying doping profile such as power FETs having a plurality of cascaded semiconductor elements each forming a single FET. Further embodiments described herein relate to methods for manufacturing semiconductor devices having a laterally varying doping profile.
One aim in the development of semiconductor devices is the increase of the blocking capability, typically denoted by BVDSS, and the reduction of the on-state resistance, typically denoted by RON or RDSON. BVDSS indicates the drain to source voltage at which a leakage current, typically denoted by IDSS, exceeds a given value, when the semiconductor device is in blocking mode. The on-state resistance RON is the resistance of the semiconductor device when operated in forward conductive mode.
Both BVDSS and RON depend on the doping concentration of the drift region. For example, the on-state resistance RON can be reduced by increasing the doping concentration. However, a high doping concentration in the drift region typically reduces the blocking capabilities of the semiconductor device.
There is therefore a desire to maintain or even improve device performance specifications.
According to an embodiment, a method for manufacturing a semiconductor device includes providing a semiconductor substrate having a first side; forming a first implantation mask having a varying thickness on the first side of the semiconductor substrate; defining regions for respective semiconductor elements in the semiconductor substrate; and implanting dopants into the semiconductor substrate through the first implantation mask to form at least a first doping region which is arranged at least partially below a first group of semiconductor elements and which has a laterally varying doping dosage and/or a laterally varying implantation depth.
According to an embodiment, a method for manufacturing a semiconductor device includes providing a semiconductor substrate having a first side; forming a source region in the semiconductor substrate at the first side of the semiconductor substrate; forming a drain region in the semiconductor substrate at the first side of the semiconductor substrate laterally spaced from the source region; forming an implantation mask having a varying thickness on the first side of the semiconductor substrate; and implanting dopants into the semiconductor substrate through the implantation mask to form a drift region which has a laterally varying doping dosage and/or a laterally varying depth between the source region and the drain region.
According to an embodiment, a semiconductor device includes a semiconductor substrate having a first side; a source metallization on the first side of the semiconductor substrate and in contact with source regions which are formed in the semiconductor substrate; a drain metallization on the first side of the semiconductor substrate and in contact with drain regions which are formed in the semiconductor substrate; and at least a first doping region formed in the semiconductor substrate, wherein the first doping region has a laterally varying doping dosage and/or a laterally varying implantation depth.
Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.
The components in the figures are not necessarily to scale, instead emphasis being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference signs designate corresponding parts. In the drawings:
In the following detailed description, reference is made to the accompanying drawings, which form a park hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top”, “bottom”, “front”, “back”, leading”, “trailing”, “lateral”, “vertical” etc., is used with reference to the orientation of the figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purpose of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. The embodiments being described use specific language, which should not be construed as limiting the scope of the appended claims.
In this specification, a second side or surface of a semiconductor substrate is considered to be formed by the lower surface or back side while a first side or first surface is considered to be formed by the top or main side or surface of the semiconductor substrate. The terms “above” and “below” as used in this specification, likewise “top” and “bottom,” therefore describe a relative location of a structural feature to another structural feature with consideration of this orientation. Furthermore, spatially relative terms such as “under”, “below”, “lower”, “over”, “upper” and the like, are used for ease of description to explain the positioning of one feature relative to a second feature. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various features, regions, sections, etc. and are also not intended to be limiting. Like terms may refer to like features throughout the description.
The terms “electrical connection” and “electrically connected” describes an ohmic connection between two features.
Herein, a “normal projection” onto a plane or surface means a perpendicular projection onto the plane or surface. In other words, the view direction is perpendicular to the surface or plane.
The semiconductor substrate can be made of any semiconductor material suitable for manufacturing semiconductor components. Examples of such materials include, without being limited thereto, elementary semiconductor materials such as silicon (Si), group IV compound semiconductor materials such as silicon carbide (SiC) or silicon germanium (SiGe), binary, ternary or quaternary III-V semiconductor materials such as gallium arsenide (GaAs), gallium phosphide (GaP), indium phosphide (InP), gallium nitride (GaN), aluminium gallium nitride (AlGaN), indium gallium phosphide (InGaPa) or indium gallium arsenide phosphide (InGaAsP), and binary or ternary II-VI semiconductor materials such as cadmium telluride (CdTe) and mercury cadmium telluride (HgCdTe) to name few. The above mentioned semiconductor materials are also referred to as homojunction semiconductor materials. When combining two different semiconductor materials a heterojunction semiconductor material is formed. Examples of heterojunction semiconductor materials include, without being limited thereto, silicon (SixC1-x) and SiGe heterojunction semiconductor material. For power semiconductor applications currently mainly Si, SiC and GaN materials are used.
N-doped regions are referred to as of first conductivity type while p-doped regions are referred to as of second conductivity type. It is, however, possible to exchange the first and second conductivity type so that the first conductivity type is p-doped and the second conductivity type is n-doped.
As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.
The thickness of the implantation mask 191 is understood to be in vertical direction perpendicular to the first side 101 of the semiconductor substrate 100. A laterally varying thickness means that the (vertical) thickness is different at different lateral locations. The implantation mask 191 can therefore include regions of different thicknesses and can also include regions of constant thickness. For example, the implantation mask 191 can include at least a first region with a laterally varying thickness and at least a second region with a constant thickness. Furthermore, the implantation mask 191 can include at least two regions with different varying thicknesses, for example regions having a different slope or gradient.
As illustrated in
The depth variation of the implantation peak is shown in
For illustrating purposes, the doping dosage in the region denoted by ΔX1 is less than the doping dosage in the region denoted by ΔX2, ΔX1 and ΔX2 indicate areas of the same size. The doping dosage is thus the number of implanted dopants per unit area in the main surface of the semiconductor substrate 100, which main surface is formed in this embodiment by the first side 101. An increasing dopant dosage also means that the total number of the implanted dopant increases in the volume of a vertical column, which is defined by the unit area and extending from the first side vertically through the semiconductor substrate 100. Such an increase, or generally the variation of the total number of implanted dopants, can be beneficially used to shape the geometrical field in the doping region 140.
As illustrated in
The photosensitive layer 190 is exposed to radiation through a grayscale mask layer 180 which has a laterally varying transmittance. The radiation can be light, such as UV light, or electron beam radiation depending on the photoresist used for the photosensitive layer 190.
In a further process, the photosensitive layer 190 is developed to form the implantation mask 191 having the laterally varying thickness. In
According to an embodiment, an implantation mask 191 with a laterally varying thickness also includes implantation masks which has a thickness modulation so that a mean thickness of the implantation mask laterally varies. For example, a thickness modulation can be obtained by forming a plurality of small trenches in the photosensitive layer 190. The trenches can have a constant width and are arranged at varying distance to each other to obtain the mean thickness variation and/or the trenches can have a varying width. In any case, a mean thickness of the implantation mask can be defined which mean thickness depends on the number and/or size of the trenches per unit area Typically, the trenches are thinner than the initial thickness of the photosensitive layer 190 to provide for a variation at small steps.
Using grayscale lithography is beneficial as only a single mask 180 is needed in comparison to approaches which use separate implantation masks to form doping regions with step-wise increasing or decreasing doping dosage. A single grayscale mask, and a photoresist material that is capable of transferring a graded exposure into a graded thickness, allows the formation of any laterally varying thickness profile so that the doping dosage and/or the implantation depth can be adjusted according to circumstances. Since only a single lithographic process is used, mask misalignments can be prevented and the manufacturing costs reduced. Since only a single implantation process is needed, the whole implantation is carried out at a given implantation dose unlike separate implantation steps which may have a varying implantation doses. Furthermore, a single implantation process requires less time.
The obtained doping dosage and/or implantation depth can be verified, for example, by a SSRM measurement (scanning spread resistance microscopy).
When a suitable voltage is applied to the gate electrode G, the enhancement transistor 231 is rendered conductive. The plurality of the depletion transistors 230a to 230d are connected in series with each other and to the enhancement transistor 231. The entirety of the depletion transistors 230a to 230d can be considered to act as a drift zone 237 of the enhancement transistor 231. In this case, the terminal D can be regarded as a drain terminal of the power semiconductor device 230. The terminal S, which is connected with the source of the enhancement transistor 231, acts as source of the semiconductor device 230.
As shown in
The semiconductor device of
The semiconductor device 230 can additionally comprise a plurality of clamping elements 233, 232a to 232d, wherein each of the clamping elements is connected in parallel to each of the transistors 231 and 230a to 230d. An overvoltage protection for the respective transistor 231 and 230a to 230d is provided by the clamping elements 233, 232a to 232d. The clamping element can be Zener diodes or other suitable elements such as PIN diodes, tunnel diodes, avalanche diodes or the like. The clamping elements 233, 232a to 232d are optional.
Each of the transistors 231, 230a to 230d is capable of blocking a given voltage such as, for example, 20 V. Due to the series connection, the total blocking voltage of the semiconductor device 230 is much larger and approximately equal to the blocking voltage of each transistor 231, 230a to 230d multiplied by the number of the transistors 231, 230a to 230d. It is thus possible to form a power semiconductor device 230 capable of blocking large voltages by a series of transistors each being capable of blocking a much lower voltage. Since the blocking voltage which each of the transistors 231, 230a to 230d has to withstand is moderate, the device requirements are not so demanding than for a single transistor which would need to block a high voltage.
The transistors 231, 230a to 230d are also referred to as semiconductor elements in other embodiments.
Each first mesa region 205 includes a plurality of second mesa regions 207 which are separated from each other by second trenches 208. The second mesa regions 207 are much smaller than the first mesa regions 205 and can be described as thin fin-shaped regions. As is apparent from
A doping region 240 is arranged below the first mesa regions 205, which doping region 240 can be considered to act as a drift zone of the semiconductor device 230 for a lateral reduction of the blocking voltage. The doping region 240 includes a plurality of sub-region 240a to 240d, each formed below a respective one of the first mesa regions 205. The doping region 240 laterally extends across the first group 235 of semiconductor elements 230a to 230d when seen in a plane projection onto the first side of the semiconductor substrate 200.
The doping region 240 can have a ring-like shape, when seen in plane projection onto the first side 201, or can have a circular shape. Other shapes are also possible.
As illustrated in
According to an embodiment, a respective sub-region 240a to 240d is assigned to a respective semiconductor element 230a to 230d. As is described in more detail below, each of the semiconductor elements 230a to 230d assumes a given electrical potential during blocking mode of the semiconductor device. The sub-regions 240a to 240d assist in the lateral relief of the blocking voltage.
For example,
The doping dosage of the doping region 240 laterally increases from sub-region to sub-region in
According to an embodiment, the doping region 240 laterally extends over two, three or more semiconductor elements 230a to 230d and has a doping dosage and/or an implantation depth which laterally increases. Typically, the lateral increase along the lateral extension of the doping region 240 is by a factor of at least two for the doping dosage and by a factor of at least two for the implantation depth.
As explained in connection with
With reference to
The first side 201 of the semiconductor substrate 200 is shown to be formed by the upper side of the second mesa regions 207a and 207b. Each of the second mesa regions 207a and 207b forms a respective fin of the semiconductor element 230b. Adjacent second mesa regions 207a and 207b are separated from each other by a respective one of the second trenches 208 and differ in their function and structure. Typically, the second mesa regions 207a and 207b forms an alternating arrangement of mesa regions 207a (first type of second mesa regions) forming source contacts 215 and mesa regions 207b (second type of second mesa regions) in which the body regions 212, drift regions 213 and drain regions 216 are formed. Two adjacent second mesa regions 207a and 207b form together a single cell of the semiconductor element 230b. Hence, each of the semiconductor elements 230a to 203d can include a plurality of transistor cells each having two second mesa regions.
The semiconductor elements can also be formed by other types of the FETs such as IGBTs. In this case, the drain region is replaced by an emitter region of opposite conductivity type.
The second mesa regions 207a (first type of second mesa regions), which form the source contacts 215, can be comprised of highly doped semiconductor material or of metal or metal alloy. The second mesa regions 207a extend from the first side 201 to respective source contact regions 214 integrated into the first mesa region 205, which source contact regions 214 are highly n-doped regions in this embodiment. The first mesa region 205 is n-doped in this embodiment and forms the source region 211.
The second mesa regions 207b (second type of second mesa regions) are comprised of semiconductor material, typically the same semiconductor material as for the first mesa region 205. The second mesa regions 207b can be formed by epitaxial deposition followed by etching. As illustrated in
Gate electrodes 221 are formed between any two adjacent second mesa regions 207a and 207b. More specifically, a gate electrode 221 is formed between a source contact 215, formed by the second mesa region 207a (first type of second mesa regions), and a semiconductor fin 207b, formed by the second mesa region 207b (second type of second mesa regions) and arranged adjacent to the source contact 215. The gate electrodes 221 are insulated from the source region 211 and the second mesa regions 207a, 207b by a gate dielectric 222.
When a voltage above a given threshold voltage is applied to the gate electrodes 221, an enhancement channel is formed in the body region 212 along the gate dielectric between the source region 211 and the drift region 213 in case of an enhancement device. In case of a depletion device, the intrinsically formed channel is depleted when the gate voltage exceeds a given threshold voltage and thus the ohmic connection between the source region 211 and the drift region 213 is interrupted.
The doping region having a laterally varying doping dosage and/or a laterally varying implantation depth is formed in the semiconductor substrate 200 below the first mesa region 205 and therefore not illustrated in
As illustrated in
Since each of the transistor cells only needs to block a comparably low voltage, such as 20 V, the blocking capabilities are not demanding. This improves the reliability of the semiconductor device 230.
With reference to
A semiconductor body 310 having a first side 301 and a second side 302 opposite the first side 301 is provided. The semiconductor material can be any of the above mentioned materials. Typically, the semiconductor body 310 is silicon wafer, a silicon carbide wafer or a gallium nitride wafer, or a composite wafer. The wafer can be supported by a non-illustrated carrier wafer which can be temporally or permanently attached to the second side 302. The semiconductor substrate can be, for example, lightly n-doped.
A first implantation mask 391 having a laterally varying vertical thickness is formed on the first side 301 of the semiconductor body 310. The first implantation mask 391 can be formed according to the processes as explained in connection with
The first implantation mask 391 has a relatively constant thickness over a central part of the semiconductor body 310 and a continuously reducing thickness towards the lateral or outer regions of the semiconductor body 310.
In a further process, as illustrated in
The implantation can occur only in deep regions of the semiconductor body 310 and/or in shallow regions. It is, for example, possible to form the first doping region 341 as a shallow region followed by an epitaxial deposition to bury the first doping region 341.
As illustrated in
The second implantation mask 392 can be formed by grayscale lithography as described above in connection with
When seen in a plan projection onto the first side 301, the second doping region 342 is surrounded by the first doping region 341 which has a laterally increasing doping dosage towards the lateral edge or rim of the semiconductor body 310. The doping dosage of the second doping region 342 is highest in its central portion and decreases from the central region towards the laterally outer edge of the second doping region 342. The doping dosage of the first doping region 341 decreases from the outside to the inside in lateral direction. The first doping region 341 can have a ring-liked shape to surround the second doping region 342 which can have a circular shape, when seen in plane projection onto the first side 301.
Typically, the first and second doping regions are of different conductivity type. Therefore, dopants of different conductivity type are used for forming the first and second doping region 341, 342. In the embodiments illustrated in
The order of the formation of the first doping region 341 and second doping region 342 can also be reversed.
In a further process, as illustrated in
For example, the enhancement device 331 can be formed in a laterally central part of the semiconductor body 310 which is ring-like surrounded by the depletion devices 330a to 330e. The ring-like arrangement is exemplified in
Alternatively, a first depletion device 330e can be formed laterally central, surrounded by the remaining depletion device 330d to 330a and the enhancement device 331 as the most outer device. In this case, the source terminal laterally surrounds the drain terminal which is central. The first doping region 341 is then p-doped and the second doping region is p-doped.
The concentric arrangement of the devices 311, 330a to 330e can be as shown in
When using N-FET devices, the doping region of the first and second doping regions 341, 341 that is formed below the drain terminal is n-doped (first conductivity type) and the respective other doping region formed below the source terminal is p-doped (second conductivity type). N-FETs include an n-doped substrate 200 or source 211 as shown in
When using P-FETs for forming the semiconductor elements, the doping relations are inversed.
The first doping region 341 is arranged to be at least partially below the first group 335 of semiconductor elements 330c to 330e. The second doping region 342 is arranged to be at least partially below the second group 336 of semiconductor elements 331, 330a to 330b.
In a further process, the first trenches 306 are filled with an insulating material 360 to improve lateral insulation.
In further processes, as exemplified in
Subsequently, gate electrodes 221 are formed between adjacent fin regions 207 followed by the formation of a first metallization 271 in electrical contact with a first group of fin regions 207a (first type of second mesa regions), and a second metallization 272 in electrical contact with a second group of fin regions 207b (second type of mesa regions). The first group of fin regions 207a forms the source contacts 215 while the second group of fin regions 207b includes the body regions 212, the drift regions 213, and the drain regions 216.
With reference to
In a further process, the second doping region 342 is formed bevor the formation of the first doping region 341 as illustrated in
In view of the above, a semiconductor substrate 200, 300 is formed which has a first side 201, 301. A source metallization 271 is formed on the first side 201, 301 of the semiconductor substrate 200, 300 and in contact with source regions 211 which are formed in the semiconductor substrate 200, 300. A drain metallization 272 is formed on the first side 201, 301 of the semiconductor substrate 200, 300 and in contact with drain regions 216 which are formed in the semiconductor substrate 200, 300. At least a first doping region 341 is formed in the semiconductor substrate 300, wherein the first doping region 341 has a laterally varying doping dosage and/or a laterally varying implantation depth.
The first doping region 341 can be arranged in the semiconductor substrate 300 at least partially below the drain regions 216 and the source regions 211.
Furthermore, a first group 335 of semiconductor elements 330e, 330d, 330c and a second group 336 of semiconductor elements 330b, 330a, 331 are at least partially formed in the semiconductor substrate 300. The first group 335 of semiconductor elements 330e, 330d, 330c and the second group 336 of semiconductor elements 330b, 330a, 331 form a plurality of semiconductor elements. A second doping region 342 of a second conductivity type can be formed in the semiconductor substrate 300 and extends at least partially below the second group 336 of semiconductor elements 330b, 330a, 331, The second doping region 342 can have a laterally varying doping concentration and/or implantation depth. The first doping region 341 is of a first conductivity type and extends at least partially below the first group 335 of semiconductor elements 330e, 330d, 330c.
The semiconductor device can include a plurality of first trenches 306 extending from the first side 301 into the semiconductor substrate 300, wherein a respective first trench 306 is arranged between respective adjacent semiconductor elements elements 330a, 330b, 330c, 330d, 330e, 331. The first doping region 341 and the second doping region 342 can extend at least partially below the first trenches 306.
The first group 335 of semiconductor elements 330e, 330d, 330c can laterally surround the second group 336 of semiconductor elements 330b, 330a, 331.
With reference to
An implantation mask 491 is formed which has a varying thickness at least above a given region of the semiconductor substrate 400 where subsequently a drift region is formed. In the present embodiment, the thickness of the implantation mask 491 reduces from the gate electrode toward a later formed drain region.
Dopants are subsequently implanted into the semiconductor substrate 400 through the implantation mask 491 to form a drift region 413, which has a laterally varying doping dosage and/or a laterally varying depth, between a source region 411 and a drain region 416 which are formed in later processes.
The source region 411 and the drain region 416 are subsequently formed in the semiconductor substrate 400 at the first side 401 of the semiconductor substrate 400. The source and drain regions 411, 416 are laterally spaced from each other with the drift region 413 arranged between the source region 411 and the drain region 416.
A body region 412 is defined between the source region 411 and the drift region 413. The drift region 413 is formed to have an increasing dopant dosage from the body region 412 to the drain region 416. The body region 412 is of opposite conductivity type to the drain region 416, source region 411 and drift region 413.
The source region 411 and the drift region 413 can also be formed prior to the formation of the drift region 413.
In further processes a source metallization is formed to be in contact with the source region 411 and a drain metallization is formed to be in contact with drain regions 416.
The lateral relief of the blocking voltage carried by the drift region is adjusted by the varying doping dosage of the drift region 413. This allows to tailoring the electrical behavior of the semiconductor device.
As described herein, any of the semiconductor devices is a so-called lateral device with source and drain metallization on the same side of the semiconductor substrate. This is beneficial as no edge termination region is needed as for vertical devices.
The semiconductor device is not limited to MOSFETs as described herein, but can includes HEMTs, JFETs, and/or IGBTs.
With the above range of variations and applications in mind, it should be understood that the present invention is not limited by the foregoing description, nor is it limited by the accompanying drawings. Instead, the present invention is limited only by the following claims and their legal equivalents.
Number | Date | Country | Kind |
---|---|---|---|
102015112729.6 | Aug 2015 | DE | national |