The present invention relates to a method for manufacturing a vertical field effect transistor structure and to a corresponding vertical field effect transistor structure.
Power MOSFETs with a vertical channel region (TMOSFETs) are typically used for the application of semiconductors having a wide band gap (e.g. silicon carbide (SiC) or gallium nitride (GaN)) in power electronics.
In the TMOSFET concept, the n+ source region located in a semiconductor material and the p channel region are interrupted by so-called trenches which extend down to the n− drift region. Within the trenches is a gate electrode which is separated from the semiconductor material by a gate oxide and serves for controlling the channel region.
Through a suitable choice of geometry, epitaxy, channel and screening doping, it is possible to optimize on-resistance, threshold voltage, short-circuit resistance, oxide load and breakdown voltage of such TMOSFETs.
The semiconductor component shown in
The semiconductor component comprises a semiconductor body 100 having an n doped first connecting zone 12, 14. This first connecting zone 12, 14 is more strongly n doped in the region of the rear side of the semiconductor body 100 and forms the n+ drain zone 12 of the MOSFET there, whereas a more weakly n doped n− drift zone 14 adjoins the n+ drain zone 12. The semiconductor body 100 further comprises a p channel zone or body zone 20 which adjoins the n− drift zone 14 and which is formed between the n− drift zone 14 and a strongly n doped second n+ connecting zone 30 formed in the region of the front side. The second n+ connecting zone 30 forms the source zone of the MOSFET.
Starting from the front side 101 of the semiconductor body 100, multiple trenches 60, two of which are shown in
In the region of the sidewalls of the trenches 60, control electrodes 40 are arranged, which connected together form the gate electrode of the MOSFET. These gate electrodes 40 are insulated from the semiconductor body 100 by a gate insulating layer 50 and run in the vertical direction of the semiconductor body from the n+ source zone 30 along the p body zone 20 to the n− drift zone 14 in order to form an electrically conducting channel in the body zone 20 along the side wall of the trench between the n+ source zone 30 and the n− drift zone 14 when a suitable control potential is applied.
The semiconductor component comprises a plurality of similar transistor structures, so-called cells, each having n+ source zones 30, p body zones 20 and gate electrodes 40, wherein a commonality of all the cells in the example is that they have an n− drift zone 14 and an n+ drain zone 12. Here, the n+ source zones 30 of all cells are electrically conductively connected to each other to form a common source zone, and the gate electrodes 40 of all cells are electrically conductively connected to each other to form a common gate electrode.
The semiconductor component shown in
The electrode 80 arranged in the trench 60 is shorted to the n+ source zone 30. To accomplish this, the electrode 80 connects directly to the n+ source zone 30 at the side walls of the trench 60 in the upper region of the trench. The electrode 80, which is preferably made of a metal or polysilicon, in particular n doped or p doped polysilicon, thus simultaneously serves as a terminal contact for the n+ source zone 30 such that, for the purposes of contacting the n+ source zones 30, this electrode 80 can be contacted directly above the trench 60, whereby it is possible to dispense with contact terminals above the semiconductor regions located between the trenches, these regions being the so-called mesa regions.
The semiconductor component further comprises strongly p doped p+ body connecting regions 22, which extend, as is clear from the perspective view in
The narrow p+ body connecting regions 22 are sufficient for connecting the p body zone 20 to the electrode 80 so as to achieve the short circuit, making the space required to do so small in the mesa region. The body diode polarity between source 30 and drain 14 created by short-circuiting the n+ source zone 30 and the p body zone 20 is the same as that of the diode of the shielding structure.
The threshold voltage of the shielding structure is set to be less than that of the body diode. When a positive voltage is applied in the source-drain direction, the majority of the current then flows through the diode of the shielding structure, which has a polarity in the forward direction, so that the cross section of the p+ body connecting regions 22 through which the p body zone 20 and the n+ source zone 30 are shorted can be small and therefore implemented in a space-saving manner. The dimensions of this silicon region between the trenches 60 can be reduced in comparison to conventional semiconductor components in this way, which helps to reduce the specific on-resistance of the semiconductor component.
When a positive drain-source voltage is applied, and when a gate potential is applied which is positive relative to a source potential, the conventional semiconductor component works like a conventional MOSFET, the circuit symbol of which is shown in
A short circuit can occur in the TMOSFET according to
Limiting the short-circuit current can be achieved using the JFET formed by the p doped zones 90, wherein the space charge zones emanating from the p doped zone 90 approach one another such that a pinch off of the short circuit current occurs. Thus, in the event of a short circuit, the p doped zones 90 function as p shielding zones.
A general optimization problem with this TMOSFET is that, in the design of each power MOSFET, a compromise must be found between low on-resistance (i.e., high current at low drain voltages) and low short-circuit current (i.e., low current at high drain voltages).
The present invention provides a vertical field effect transistor structure and a method for manufacturing a vertical field effect transistor structure.
Preferred further developments of the present invention are disclosed herein.
An underlying feature of the present invention is that the p body connection is designed deeper than the p body region, i.e., it extends into the n− drift zone. Thus, a p-n junction is created below the channel, which reduces the resistance at high drain voltages and thus helps to reduce the short-circuit current. At high drain voltages, a depletion zone thus forms in the n− drift zone, which causes an increase in the resistance of the component. In the event of a short circuit, it is precisely this increase in the resistance that helps to limit the short-circuit current.
The trenches are preferably widened by cyclic oxidation and oxide etching so that the mesas located between the trenches are narrowed down to fins.
According to a preferred further development of the present invention, the reverse current path runs inside the trenches, a respective electrode being arranged in the trenches which is electrically conductively connected to the second connecting zone and which is electrically insulated from the control electrode, and which contacts the doped zone of the second conductivity type at the bottom of the trenches.
In accordance with a further preferred development of the present invention, the body connecting regions of the second conductivity type electrically contact the doped zones of the second conductivity type, wherein the reverse current path runs through the body connecting regions of the second conductivity type and through the doped zones of the second conductivity type. This has the advantage that complex processing of the connection in the trenches for purposes of producing the electrodes is not necessary.
According to a further preferred development of the present invention, the first connecting zone has a less doped drift region and a more doped drain region of the first conductivity type, the doped zones of the second conductivity type are arranged in the drift region, and the body connecting regions of the second conductivity type extend into the drift region.
According to a further preferred development of the present invention, a spreading zone of the first conductivity type is provided between the first connection region and the channel zone. This improves current distribution.
According to another preferred further development of the present invention, the semiconductor body is made of silicon carbide or gallium nitride.
Further features and advantages of the present invention are explained below by means of example embodiments with reference to the figures.
In the figures, identical reference numbers denote identical or functionally identical elements.
The process state according to
According to
In contrast to the conventional structure according to
Also, the p+ doped regions 22′ are implanted substantially deeper than the p+ doped regions 22 according to
By way of an annealing step, the p doped zones 90 and the p+ doped regions 22′ can be diffused out and activated.
Referring further to
Subsequently, according to
These gate electrodes 40 can be produced by a so-called polyspacer process, for example. For this purpose, using an anisotropic etching process, for example, the polysilicon layer 40 is etched back until the polysilicon layer 40 is removed at the bottom of the trenches 60′, from the front side 101 of the semiconductor body 100, and partially from the side walls in the upper region of the widened trenches 60′. The gate insulating layer 50 is also removed from the front side 101.
Finally, an insulation layer 70, for example an oxide layer, is generated on the exposed regions of the gate electrodes 60. To this end, either the insulation layer 70 is deposited onto the gate electrodes 40 or the gate electrodes 40 are subjected to an oxidation process. Subsequently, the insulation layer 70 is removed from the front side 101 of the semiconductor body 100 and in the bottom region of the widened trenches 60′
Subsequently, the widened trenches 60′ are filled with an electrode material, for example a metal or polysilicon, for purposes of manufacturing the electrodes 80, as shown in
Advantageously, if the electrodes are made of a metal or n doped silicon, a silicide is advantageously applied to the exposed front side 101 of the semiconductor body 100, at least in the region of the p doped zone, prior to the manufacture of the electrodes 80 in order to obtain good ohmic contact between the electrode 80 and the p doped zone 90 to prevent a p-n junction or Schottky contact from developing at this junction. The contacting of the gate electrodes 40 can occur as in conventional trench transistors, and is not shown here.
The process sequence described above focuses solely on the processes in the cell field. Other processes outside the cell field, such as edge termination, contact pad lead-outs, etc., must be considered. In addition, each step can include multiple sub-steps that are not specifically listed.
In contrast to
Thus, the trenches 60′ according to
Although the present invention has been described by means of preferred embodiment examples, it is not limited thereto. In particular, the materials and topologies mentioned are only exemplary and not limited to the examples explained. The geometries shown are also only exemplary and can be arbitrarily varied as needed.
In the embodiments described above, although the p+ doped regions and the p doped zones were formed in a common implantation step, it is also possible to use two separate implantation steps for this purpose.
Number | Date | Country | Kind |
---|---|---|---|
10 2022 210 835.3 | Oct 2022 | DE | national |