The present invention relates to a semiconductor device, and more particularly to a silicon carbide semiconductor device and a method for manufacturing the same.
Wide bandgap semiconductors are drawing public attention as semiconductor materials of semiconductor devices whose breakdown voltage is high and which are capable of conducting large currents therethrough (power devices). Among other wide bandgap semiconductors, silicon carbide (SiC) has a particularly high breakdown electric field, and is therefore expected as a semiconductor that is most suitable for next-generation low-loss power devices. Because good-quality silicon dioxide (SiO2) films can be formed through thermal oxidation on SiC, insulated gate-type SiC-power MOSFETs using such silicon dioxide films as gate insulating films have been developed.
When an SiC-power MOSFET is manufactured, the conductivity of the semiconductor is controlled by using an ion implantation method. In the process, it is necessary to perform an activation annealing step of subjecting SiC to a heat treatment at a high temperature so as to restore crystal defects and activate impurities. Particularly, where aluminum ions are implanted into silicon carbide in order to form p-type impurity regions, it is necessary to subject a silicon carbide substrate to a heat treatment at a high temperature exceeding 1600° C. in order to restore the crystal structure.
However, even if a heat treatment is performed at such a high temperature, the crystallinity of the silicon carbide semiconductor is not completely restored, with disturbance of crystallinity remaining in some parts. As a result, if a gate insulating film is formed through thermal oxidation on a substrate with disturbed crystallinity, it is not possible to obtain a desirable SiO2/SiC interface.
Specifically, the channel mobility lowers. Therefore, the channel resistance of the SiC-power MOSFET increases, thus failing to sufficiently bring out the low-loss property which SiC naturally has. Since it is difficult to obtain an oxide film or an SiO2/SiC interface which satisfies predetermined characteristics, the production yield of the oxide film lowers extremely. Such a problem is not limited to aluminum ions, but it similarly occurs also when boron or another p-type impurity is used.
In order to solve this problem, Patent Document No. 1 proposes a vertical SiC-power MOSFET having a structure shown in
A channel layer 27 is provided on the surface of the drift layer 3 so as to connect together the source regions 5. The channel layer 27 includes a boundary portion 27a in the vicinity of a gate insulating film 8a and a boundary portion 27b in the vicinity of the drift layer 3, with the impurity concentration of the boundary portion 27a being lower than the impurity concentration of the boundary portion 27b.
According to Patent Document No. 1, the channel resistance of the vertical SiC-power MOSFET in the ON state includes the accumulation channel resistance (Rchannel) formed in the channel layer, and the accumulation drift resistance (the internal resistance, Racc-drift) in the channel layer. With the structure of
On the other hand, it is required of an SiC-power MOSFET that the reliability of the gate insulating film in the OFF state is sufficiently high. With an SiC-power MOSFET, if a high voltage is applied to the drain electrode in the OFF state, a high electric field is applied to the gate insulating film over the area between the wells. Particularly, an electric field of the highest intensity is applied to the gate insulating film above a point R that is located in the middle between the wells 4a shown in
Patent Document No. 2 discloses a technique of suppressing the localization of electric field at the point R by providing an accumulated channel, i.e., an n-type channel region 28, over the p-type well 4a while providing no high-concentration n-type impurity region in the vicinity of the surface layer of the drift layer 3 between the wells 4a, as shown in
However, a study by the present inventor has revealed that, if the boundary portion 27b having a high impurity concentration is provided on the surface of the drift layer 3 as shown in
Specifically, with the structure shown in
If the boundary layer 27b which is an n-type impurity region having such a high concentration is provided on the drift layer 3 between the wells 4a, a high electric field localizes at the boundary layer 27b when a high voltage is applied to a drain electrode 1. As a result, avalanche breakdown occurs, and particularly with a MOSFET of a short gate length having a gate length shorter than 1 μm, the short channel effect is likely to occur because the voltage barrier of the source drops trailing the drain electric field. Therefore, the drain leak increases, and the threshold voltage Vth of the device decreases. If the electric field applied to the boundary layer 27b increases, the electric field intensity applied to the gate insulating film located directly above the boundary layer 27b also increases, thereby causing problems such as an increase in the gate leak or breakdown of the gate insulating film. Particularly, the localization of the electric field at the point R of the gate insulating film becomes pronounced.
On the other hand, in a case where no n-type impurity region is formed over the drift layer 3 between the wells 4a as with the structure of the MOSFET shown in
Moreover, since the concentration of the wells 4a is higher than the drift layer 3, a depletion layer 3d is formed in the drift layer 3 even when the MOSFET is in the ON state. Therefore, as indicated by arrows in
The present invention has been made in order to solve at least one of such problems with the conventional techniques, and has an object to provide a wide bandgap semiconductor device with which it is possible to reduce the channel resistance in the ON state and to improve the breakdown voltage in the OFF state, thus improving the reliability.
A semiconductor device of the present invention includes a plurality of unit cells arranged at least one-dimensionally, each unit cell including: a substrate made of an n-type wide bandgap semiconductor; a drift layer formed on the substrate and made of the n-type wide bandgap semiconductor; a p-type well provided in the drift layer; a first n-type impurity region provided in the well; a surface channel layer formed at least on a surface of the well so as to connect together the first n-type impurity region and the drift layer; a second n-type impurity region provided in a surface region of the well which is under the surface channel layer and which spans the first n-type impurity region and the drift layer, the second n-type impurity region having an impurity concentration generally equal to or greater than an impurity concentration of the well; a third n-type impurity region formed in a surface region of the drift layer adjacent to the second n-type impurity region; a gate insulating film formed on the surface channel layer; a gate electrode formed on the gate insulating film; a source electrode electrically connected to the first n-type impurity region; and a drain electrode provided on one surface of the substrate which is opposite to a surface thereof on which the drift layer is formed.
In a preferred embodiment, the surface channel layer contains an n-type impurity.
In a preferred embodiment, the surface channel layer contains a p-type impurity.
In a preferred embodiment, an impurity concentration of the n-type impurity or the p-type impurity is 1×1016 cm−3 or less.
In a preferred embodiment, each unit cell includes a fourth n-type impurity region formed in a surface region of the drift layer between the third n-type impurity region thereof and a third n-type impurity region of an adjacent unit cell; and an impurity concentration of the fourth n-type impurity region is lower than an impurity concentration of the third n-type impurity region and is generally equal to or greater than an impurity concentration of the drift layer.
In a preferred embodiment, the semiconductor device further includes a fifth n-type impurity region formed at a position in the drift layer that is adjacent to the fourth n-type impurity region and that includes an apex of the unit cell; and an impurity concentration of the fifth n-type impurity region is lower than the impurity concentration of the fourth n-type impurity region.
In a preferred embodiment, as each unit cell is seen from a surface side of the drift layer, the well has a generally rectangular shape, and the third n-type impurity region is not provided at corners of the rectangular shape of the well.
In a preferred embodiment, as each unit cell is seen from a surface side of the drift layer, the third n-type impurity region continuously surrounds the well.
In a preferred embodiment, a depth of the third n-type impurity region is smaller than a depth of the first n-type impurity region.
In a preferred embodiment, a depth of the third n-type impurity region is smaller than a width of the second n-type impurity region in a direction in which the plurality of unit cells are arranged.
In a preferred embodiment, a depth of the third n-type impurity region is smaller than a depth of the well.
In a preferred embodiment, an expression:
is satisfied, where Next denotes an impurity concentration of the third n-type impurity region, Na denotes the impurity concentration of the well, ∈ denotes a relative dielectric constant of silicon carbide, q denotes an elementary electric charge, Vbi denotes an internal potential of a junction portion between the second n-type impurity region and the third n-type impurity region, and Lg denotes a channel length of a channel formed in the surface channel layer.
In a preferred embodiment, an impurity concentration of the third n-type impurity region gradually decreases away from the second n-type impurity region in a direction in which the plurality of unit cells are arranged.
In a preferred embodiment, a concentration of the third n-type impurity region gradually decreases away from a surface of the drift layer.
A method for manufacturing a semiconductor device of the present invention includes the steps of: (A) preparing a substrate made of an n-type wide bandgap semiconductor in which a drift layer made of an n-type wide bandgap semiconductor is provided; (B) forming a well mask on the drift layer; (C) forming a p-type well in the drift layer by implanting a p-type impurity using the well mask; (D) implanting an n-type impurity using the well mask from a vertical direction and from an inclined direction with respect to the substrate, thereby forming an impurity region in the drift layer, the impurity region including a region to be a first n-type impurity region and a second n-type impurity region, and forming a third n-type impurity region in a portion of the drift layer under the well mask; (E) forming a first n-type impurity region mask on the drift layer in a self-aligned manner with respect to the well mask; (F) implanting an n-type impurity using the first n-type impurity region mask, thereby forming the first n-type impurity region in the drift layer, thus delimiting the second n-type impurity region; (G) removing the first n-type impurity region mask and the well mask; (H) performing an activation annealing process on the drift layer; (I) forming a surface channel layer having a low impurity concentration by epitaxial growth on the second n-type impurity region and the third n-type impurity region so as to be in contact with the first n-type impurity region and the well; (J) forming a gate insulating film on a surface of the surface channel layer; (K) forming a gate electrode on the gate insulating film; and (L) forming a source electrode and a drain electrode so as to be in contact with the first n-type impurity region and the substrate, respectively.
In a preferred embodiment, in the step (D), the third n-type impurity region is formed in the portion of the drift layer under the well mask by implanting the n-type impurity from a direction inclined with respect to the substrate within a plane perpendicular to a side that defines an opening shape of the well mask.
In a preferred embodiment, in the step (D), the third n-type impurity region is formed in the portion of the drift layer under the well mask by continuously rotating the substrate while implanting the n-type impurity from a direction inclined with respect to the substrate.
In a preferred embodiment, in the step (I), the surface channel layer is formed while an impurity gas other than a material gas of SiC is not intentionally supplied.
In a preferred embodiment, in the step (D), the third n-type impurity region is formed in the portion of the drift layer under the well mask by rotating the substrate stepwise while implanting the n-type impurity from a direction inclined with respect to the substrate. More specifically, the n-type impurity is implanted into the substrate while supporting the substrate so that the normal is unparallel to the direction in which the ion beam is applied, after which the substrate is rotated by θ=360°/n (n is an integer of 2 or more) using the normal as the axis. Then, the n-type impurity is implanted into the substrate, and the substrate is rotated by θ=360°/n (n is an integer of 2 or more) using the normal as the axis. By performing the implantation n times while rotating the substrate (n−1) times, the third n-type impurity region is formed in a portion of the drift layer under the well mask. The implantation may be performed more than n times, and the substrate may be rotated more than (n−1) times.
In a preferred embodiment, in the step (I), the surface channel layer is formed while a material gas of SiC and a gas to be an n-type impurity or p-type impurity are supplied.
According to the present invention, since the third n-type impurity region is provided, the depletion layer which is formed in the drift layer by the contact with the well does not extend to the position where the third n-type impurity region is provided because of the carrier supplied from the third n-type impurity region. Therefore, the channel length does not extend, and electrons can flow into the drift layer through the third n-type impurity region. Thus, the channel resistance is effectively reduced. Since the surface channel layer is provided, there is substantially no disturbance of crystallinity in the vicinity of the interface between the surface channel layer and the gate insulating film, and the channel resistance is low.
Since the fourth n-type impurity region is provided, it is possible to suppress the localization of an electric field in the gate insulating film at a position in the middle between wells because of the voltage applied to the drain electrode while the semiconductor device is in the OFF state, and it is possible to improve the breakdown voltage and improve the reliability.
a) to 3(l) are cross-sectional views showing steps of a method for manufacturing the semiconductor device shown in
a) to 4(c) are diagrams showing an ion implantation step for forming a third n-type impurity region.
a) to 6(i) are cross-sectional views showing steps of a method for manufacturing the semiconductor device shown in
a) is a cross-sectional view showing a semiconductor device according to a second embodiment of the present invention, and
A semiconductor device according to a first embodiment of the present invention will now be described. In the present embodiment, the present invention will be described with reference to an example of a double implanted MOSFET.
The unit cell U of the MOSFET 101 includes a substrate 2 made of a wide bandgap semiconductor, and the drift layer 3 made of a wide bandgap semiconductor formed on the substrate 2. In the present specification, a wide bandgap semiconductor refers to a semiconductor made of SiC, GaN, or the like. In the present embodiment, the substrate 2 is, for example, a low-resistance SiC substrate containing 1×1018 cm−3 or more of an n-type impurity (nitrogen, phosphorus, arsenic, or the like). The drift layer 3 is an SiC semiconductor layer doped with about 1×1014 cm−3 or more and about 1×1016 cm−3 or less of a p-type impurity (e.g., aluminum). The drift layer 3 can be formed for example through epitaxial growth by a CVD method, or the like, on the substrate 2.
In a portion of the drift layer 3, the p-type wells 4a are provided so as to extend from the surface toward the inside of the drift layer 3. The well 4a is doped with, for example, 1×1016 cm−3 or more and 1×1018 cm−3 or less of a p-type impurity. In order to realize a high breakdown voltage, the concentration of the well 4a is preferably 1×1017 cm−3 or more and 1×1019 cm−3 or less.
In a portion of the well 4a, a p+-type contact region 4b and a source region 5 which is a first n-type impurity region are formed so as to be adjacent to each other. The contact region 4b and the source region 5 are formed so as to extend from the surface toward the inside of the well 4a. The p+-type contact region 4b is doped with about 5×1019 cm−3 of a p-type impurity, and the source region 5 is doped with 1×1019 cm−3 or more and 1×1020 cm−3 or less of an n-type impurity.
A surface channel layer 7b is formed at least on the surface of the well 4a so as to connect together the source region 5 and the drift layer 3. The surface channel layer 7b has the n-type conductivity, and contains a slight amount of at least one of n-type nitrogen, phosphorus and antimony. The impurity concentration is preferably low, and is preferably lower than the amount of intentional doping. For example, it is preferably about an amount by which nitrogen, phosphorus and antimony contained in the background atmosphere, or the like, are inadvertently taken in during epitaxial crystal growth by a CVD method, or the like. With such a surface channel layer 7b, the threshold voltage of the MOSFET is no longer substantially dependent on the impurity concentration of the surface channel layer. When not intentionally doped with an n-type impurity, the impurity concentration of a surface channel layer 7 is preferably 1×1016 cm−3 or less, though it also depends on the growth conditions. It is more preferred that the impurity concentration of the surface channel layer 7 can be suppressed to be 1×1015 cm−3 or less. Note however that the impurity concentration of the surface channel layer 7 being 1×1014 cm−3 or less is not preferable because the channel layer itself then becomes a high-resistance layer, and in such a case it is preferably set in a concentration range to be shown below.
Where the surface channel layer 7b is formed by intentionally doping it with an n-type impurity, the impurity concentration is preferably 1×1014 cm−3 or more and 1×1016 cm−3 or less taking into consideration the stability of the threshold voltage, i.e., so that the impurity concentration can be controlled appropriately. Since the impurity concentration of the well 4a is typically set to 1×1017 cm−3 or more, the impurity concentration of the surface channel layer 7b can reliably be made lower than the impurity concentration of the well 4a so that the threshold voltage is unlikely to fluctuate by setting the impurity concentration of the surface channel layer 7b to 1×1016 cm−3 or less, which is smaller than the impurity concentration of the well 4a by an order of magnitude or more. If the impurity concentration of the surface channel layer 7b is 1×1014 cm−3 or more, it is possible to suppress the resistance with the semiconductor region connected to the surface channel layer 7b to such a small value that it is virtually negligible.
The surface channel layer 7b has the p-type conductivity, and may contain a slight amount of at least one of boron and aluminum. Also in such a case, the impurity concentration is preferably low, and is preferably lower than the amount of intentional doping. With such a surface channel layer 7b, the threshold voltage of the MOSFET is no longer substantially dependent on the impurity concentration of the surface channel layer. When not intentionally doped with an impurity, the p-type impurity concentration of the surface channel layer 7b is preferably 1×1016 cm−3 or less.
Alternatively, the surface channel layer 7b may be formed by intentionally doping it with a p-type impurity. In such a case, the impurity concentration is preferably 1×1014 cm−3 or more and 1×1016 cm−3 or less taking into consideration the stability of the threshold voltage. As with n-type, the threshold voltage is unlikely to fluctuate if the impurity concentration of the surface channel layer 7b is 1×1016 cm−3 or less. The p-type impurity concentration of the surface channel layer 7b is preferably as low as possible because then the carrier scattering decreases and the channel mobility increases. While the lower limit of the p-type impurity concentration depends on the crystal growth facility, the channel mobility is saturated at about 1×1014 cm−3. Therefore, the impurity concentration may be 1×1014 cm−3 or more.
When the surface channel layer 7b is of the p-type, it forms a pn junction with the source region 5, thus making it more difficult for the current to flow from the source region 5 to the surface of the surface channel layer 7b. A pn junction is formed also between the surface channel layer 7b and the wells 4a. Therefore, it is preferred to form an n-type impurity region by ion implantation, or the like, in the surface channel layer 7b so that the source region 5 and the wells 4a are connected with the vicinity of the surface of the surface channel layer 7b by the n-type impurity region formed.
A second n-type impurity region 7a is provided in a surface region of the well 4a which is under the surface channel layer 7b and which spans the source region 5 and the drift layer 3. It is preferred that the impurity concentration of the second n-type impurity region 7a is generally equal to or greater than the impurity concentration of the well 4a. Specifically, where the impurity concentration of the well 4a is about 1×1017 cm−3, which is a typical value, the threshold voltage can be controlled to about 4 V, which is an appropriate value, by adjusting the impurity concentration of the second impurity region 7a to about 1.5×1017 cm−3. Where the impurity concentration of the well 4a is about 1×1019 cm−3, the threshold voltage can be controlled to about 4 V by adjusting the impurity concentration of the second impurity region 7a to about 5×1018 cm−3. Thus, when the impurity concentration range of the well 4a is taken into consideration, the threshold voltage can be controlled to about 4 V by adjusting the impurity concentration of the second impurity region 7a to 5×1016 cm−3 or more and 1×1019 cm−3 or less. Also when it is desirable to control the threshold voltage to a value that is slightly lower or higher than 4 V, it is possible to realize an intended threshold value by adjusting the impurity concentration of the second impurity region 7a within this range.
The second n-type impurity region 7a and the surface channel layer 7b together form the channel 7. The thickness of the surface channel layer 7b is preferably 10 nm or more and 200 nm or less. The threshold value of the MOSFET 101 is controlled substantially by the impurity concentration, the thickness or more essentially the dose of the second impurity region 7a. Note however that the influence of the thickness of the surface channel layer 7b on the threshold value is smaller than that of the concentration. The thickness of the surface channel layer 7b is greatly restricted by the manufacturing process of the MOSFET 101.
If the surface channel layer 7b has a thickness of about 10 nm before the formation of the gate oxide film, an interface between the gate oxide film and the surface channel layer and a surface of the gate oxide film, which are ideally smooth, can be obtained between the gate oxide film and the surface channel layer 7b. However, if the thickness of the surface channel layer 7b is smaller than 10 nm, it is difficult to obtain a smooth interface or gate oxide film surface. Where the thickness of the surface channel layer 7b is 200 nm or more, the electric field on the drain side seeps into the surface channel layer 7b, adversely influencing the channel modulation. Specifically, the short channel effect is pronounced.
Therefore, also taking into consideration the process margin, it is preferred that the thickness of the surface channel layer 7b is 30 nm or more and 100 nm or less. With the thickness in this range, it is possible to stably manufacture the MOSFET 101 having predetermined characteristics even if errors caused by the manufacturing process are taken into consideration.
It is preferred that the sheet concentration of the second n-type impurity region 7a is 1012 cm−2. The threshold voltage of the MOSFET 101 can also be controlled by performing concentration control of the second n-type impurity region 7a. For example, where the thickness of the surface channel layer 7b is set to 50 nm, the threshold voltage can be controlled to 3 V or more and 6 V or less by changing the sheet concentration of the second n-type impurity region 7a in the range of 1×1012 cm−2 or more and 5×1012 cm−2 or less. By using ion implantation, the fluctuation of the impurity concentration of the second n-type impurity region 7a can be suppressed to 1% or less, and it is therefore possible to control the threshold voltage with high precision.
Thus, the source region 5, the contact region 4b and the second n-type impurity region 7a are formed in the wells 4a.
As shown in
Each unit cell U includes a fourth n-type impurity region 7d formed in the surface region of the drift layer 3 between the third n-type impurity region 7c thereof and the third n-type impurity region 7c of an adjacent unit cell U. It is preferred that the impurity concentration of the fourth n-type impurity region 7d is lower than the impurity concentration of the third n-type impurity region 7c and is generally equal to or greater than the impurity concentration of the drift layer 3.
As shown in
The gate insulating film 8a is provided on the surface channel layer 7b. A gate electrode 8b is provided on the gate insulating film 8a. The gate insulating film 8a is made of silicon oxide, for example, and may be formed by depositing and patterning silicon oxide or performing thermal oxidation on the surface of the surface channel layer 7b. The gate electrode 8b is made of polysilicon, for example.
The source electrode 6 is provided so as to be electrically connected to the source region 5 and the contact region 4b. The drain electrode 1 is provided on one surface of the substrate 2 on which the drift layer 3 is not provided. The source electrode 6 and the drain electrode 1 are made of an Ni alloy, for example, and is subjected to a heat treatment so as to be in ohmic contact with the source region 5, the contact region 4b and the substrate 2.
An interlayer insulating film 9 is provided so as to cover the gate electrode 8b, and a contact is formed in the interlayer insulating film 9 so that the source electrode is exposed therethrough. The source electrode 6 is electrically connected to a source line 10. The source electrodes 6 of other unit cells are also connected to the source line 10.
In each unit cell of the MOSFET 101 having such a configuration, if a bias voltage greater than or equal to the threshold voltage is applied to the gate electrode 8b in the presence of a predetermined voltage applied between the source electrode 6 and the drain electrode 1, electrons, which are carriers, travel from the source electrode 6 through the source region 5 and through the surface channel layer 7b in the vicinity of the interface between the surface channel layer 7b and the gate insulating film 8a, as indicated by arrows in
With the provision of the third n-type impurity region 7c, the depletion layer 3d which is formed in the drift layer 3 through contact with the well 4a is prevented by the carriers supplied from the third n-type impurity region 7c from extending to the position where the third n-type impurity region 7c is provided. Therefore, electrons traveling through the surface channel layer 7b can flow into the drift layer 3 through the third n-type impurity region 7c, and therefore the channel length does not extend as described above with reference to
Each unit cell U includes the fourth n-type impurity region 7d which is formed in the surface region of the drift layer 3 between the third n-type impurity region 7c thereof and the third n-type impurity region 7c of an adjacent unit cell U. The impurity concentration of the fourth n-type impurity region 7d is lower than the impurity concentration of the third n-type impurity region 7c. This suppresses the problem that the depletion layers extending from adjacent wells 4a into the drift layer 3 reach a position below a point P in the middle between the wells 4a, thereby localizing the voltage applied to the drain electrode 1 at the point P while the MOSFET 101 is in the OFF state. Therefore, it is possible to improve the breakdown voltage of the MOSFET 101 and improve the reliability.
In order to reduce the accumulation drift resistance of the channel in the MOSFET 101 while exerting such effects as described above, it is preferred that the shape and the impurity concentration of the third n-type impurity region 7c satisfy predetermined conditions. Specifically, the depth d7c of the third n-type impurity region 7c is preferably smaller than the depth d4a of the well 4a as shown in
A primary role of the third n-type impurity region 7c lies in the reduction of the accumulation drift resistance, and it is possible to reduce the accumulation drift resistance by increasing the impurity concentration in the vicinity of the surface channel layer 7b. Therefore, it is possible to obtain the effect of reducing the accumulation drift resistance whether the depth of the third n-type impurity region 7c is small or large. However, the OFF-state characteristics can be improved by setting the depth d7c of the third n-type impurity region 7c to be smaller than the depth d4a of the well 4a. Specifically, it is possible to improve the reliability in the gate insulating film 8a in the presence of a high voltage applied to the drain electrode 1, and to suppress problems such as the short channel effect and an increase in the drain leak due to a high electric field applied to the drain. Improving these characteristics is typically in a trade-off relationship with the reduction of the accumulation drift resistance. However, it is possible to achieve the reduction of the accumulation drift resistance and the improvement of these characteristics by reducing the depth d7c of the third n-type impurity region 7c so that a high voltage is not applied to the gate insulating film 8a in the OFF state of the MOSFET 101.
Moreover, it is preferred that the impurity concentration of the third n-type impurity region 7c gradually decreases away from the second n-type impurity region 7a in the direction in which the unit cells U are arranged. It is also preferred that the impurity concentration of the third n-type impurity region 7c gradually decreases away from the surface of the drift layer (toward the inside of the drift layer 3). Thus, the electric field intensity at the point P shown in
The depth d7c of the third n-type impurity region 7c is generally equal to the width w7c of the third n-type impurity region in the direction in which the unit cells U are arranged, though it also depends on the method (process) of forming the third n-type impurity region 7c. That is, where an impurity region on an order of magnitude smaller than 1 μm is formed by implanting an impurity into a silicon carbide semiconductor, the depth of the impurity region to be formed and the horizontal extent thereof will be generally equal to each other.
Therefore, it is possible to reduce the width w7c of the third impurity region by reducing the depth d7c of the third impurity region 7c. Since the width w7a of the second n-type impurity region 7a in the direction in which the unit cells U are arranged is the channel length (Lg) of the MOSFET 101, it is possible to effectively reduce the accumulation drift resistance by setting the width w7c of the third impurity region, i.e., the depth d7c of the third impurity region 7c, to be smaller than the width w7a of the second n-type impurity region 7a in the direction in which the unit cells U are arranged.
It is preferred that the depth d7c of the third n-type impurity region 7c is smaller than the depth d5 of the first n-type impurity region 5. The third n-type impurity region 7c is formed by ion implantation because the carrier concentration of the well 4a may be influenced and the breakdown voltage, or the like, may be adversely influenced if the third n-type impurity region 7c is designed to be deeper than the first n-type impurity region 5.
It is preferred that the relationship below is satisfied, where Next denotes the impurity concentration of the third n-type impurity region 7c, Na denotes the impurity concentration of the well 4a, ∈ denotes the relative dielectric constant of silicon carbide, q denotes the elementary electric charge, Vbi (built-in potential) denotes the internal potential of the junction portion between the second n-type impurity region 7a and the third n-type impurity region 7c, and Lg denotes the channel length of the channel formed in the surface channel layer 7b.
By controlling the impurity concentration Next of the third n-type impurity region 7c so as to satisfy the relationship of the expression above, it is possible to optimize the reduction of the accumulation drift resistance and the suppression of the electric field localization between JFETs, particularly at the point R, which are in a trade-off relationship.
For example, the MOSFET 101 can be manufactured by the following method. First, as shown in
Next, as shown in
Next, as shown in
In this process, the substrate 2 may be continuously rotated using the normal as the rotation axis while the impurity is injected from a direction inclined with respect to the surface of the drift layer 3 as shown in
Although the substrate temperature is preferably kept at a high temperature during implantation as described above, it may be difficult to continuously rotate the substrate while heating the substrate depending on the substrate heating scheme. In such a case, the substrate 2 may be rotated stepwise while implanting the impurity into the drift layer 3 from a direction inclined with respect to the surface of the drift layer 3 of the substrate 2. More specifically, an n-type impurity is implanted into the drift layer 3 of the substrate 2 while supporting the substrate 2 so that the normal is unparallel to the direction in which the impurity ions are applied, after which the substrate is rotated by θ=360°/n (n is an integer of 2 or more) using the normal as the axis. Then, an n-type impurity is implanted into the drift layer 3, and the substrate 2 is rotated by θ=360°/n (n is an integer of 2 or more) using the normal as the axis. By performing the implantation n times while rotating the substrate 2 (n−1) times, the third n-type impurity region 7c is formed in a portion of the drift layer 3 under the well mask 50. The implantation may be performed more than n times, and the substrate may be rotated more than (n−1) times.
Then, as shown in
Thus, by implanting in a self-aligned manner using the well mask 50 for the threshold voltage control, the implantation species is not implanted into the fourth n-type impurity region 7d. Therefore, it is possible to suppress the localization of the electric field at the fourth n-type impurity region 7d which increases the drain leak while the MOSFET is in the OFF state, and it is also possible to suppress the decrease in the breakdown voltage. Moreover, it is possible to suppress the increase in the leak through the gate insulating film or the breakdown of the gate insulating film due to a high drain electric field, and it is also possible to suppress the decrease in the threshold voltage.
Then, a mask for the source region 5 is formed. As shown in
The resist mask 53 is removed as shown in
Then, the well mask 50 and the mask 52 are removed and a mask 54 is formed which defines the contact region 4b as shown in
Note that when the impurity concentration of the well 4 increases due to the design and the dose of the ion implantation (
Also when the n-type impurity region is not etched, the depth of the second impurity region 7a is preferably shallower than the contact region 4b. In other words, the contact region 4b is preferably deeper than the second impurity region 7a. Then, the contact region 4b can contact the well 4a in an un-counter-doped region, and it is therefore possible to reduce the contact resistance to the well 4a.
After the mask 54 is removed, activation annealing is performed by holding the substrate 2 in an atmosphere of an inert gas such as argon at 1700° C. for 30 min in order to activate the impurity implanted into the drift layer 3. In this process, macrosteps whose height is about 10 nm or more and about 100 nm or less occur on the drift layer 3, thereby increasing the surface roughness and deteriorating the surface smoothness. Therefore, in order to prevent the deterioration of the surface flatness, it is preferred that the heat treatment is performed while the surface of the drift layer 3 is covered with a material which withstands high temperatures such as DLC (diamond-like carbon). Thus, the surface roughness can be suppressed to about 1 nm or more and about 10 nm or less.
Next, as shown in
Then, as shown in
Next, as shown in
Then, as shown in
As shown in
Note that in the present embodiment, the impurity concentration of the fourth n-type impurity region 7d is equal to the impurity concentration of the drift layer 3. Where the impurity concentration of the fourth n-type impurity region 7d is set to be higher than the impurity concentration of the drift layer 3, it is preferred that the impurity concentration of the fourth n-type impurity region 7d is determined so that the reliability of the gate oxide film at the middle point between the p-type wells 4a of adjacent cells is ensured while the MOSFET 101 is in the OFF state and the drain voltage is maintained.
A semiconductor device according to a second embodiment of the present invention will now be described.
As shown in
Repeating a statement above, the impurity concentration of the fourth n-type impurity region 7d is set to be lower than the impurity concentration of the third n-type impurity region 7c. After effectively reducing the channel resistance by setting the concentration as described above, it is possible to improve the reliability at the point P in the middle between the wells 4. That is, it is possible to effectively avoid the electric field localization at the point P which occurs when a large voltage is applied to the drain.
As shown in
Therefore, in the present embodiment, the impurity concentration of the fifth n-type impurity region 31 is set to be smaller than the fourth n-type impurity region 7d. More preferably, the impurity concentration of the fifth n-type impurity region is set to be smaller than the concentration of the fourth impurity region so that the fifth n-type impurity region is depleted before the fourth n-type impurity region is depleted when a voltage is applied to the drain electrode 1 of the MOSFET 102. Therefore, even if the distance between the wells 4a of two adjacent unit cells U is long at a position passing through the point Q which is an apex of a unit cell U (the position of line 7A-7A), when the drift layer 3 is depleted at the position where adjacent unit cells U are in contact with each other along a side thereof, the drift layer 3 can be depleted also in the vicinity of the point Q which is an apex of a unit cell U. Therefore, it is possible to suppress the localization of an electric field at the point Q while the MOSFET 102 is in the OFF state and the drain voltage is applied. As a result, it is possible to suppress the increase in the drain leak in the OFF state, and to suppress the occurrence of a decrease in the breakdown voltage. Moreover, it is possible to suppress an increase in the leak through the gate insulating film or the breakdown of the gate insulating film due to a high drain electric field, and it is also possible to suppress the decrease in the threshold voltage.
The results of an experiment on how the channel resistance is influenced when the impurity concentrations of the third n-type impurity region and the well are varied in the MOSFET 101 of the first embodiment will now be described.
As shown in
As can be seen from
In contrast, if the impurity concentration Next of the third n-type impurity region 7c is similar to that of the drift layer 3, i.e., in the range of 5×1015 cm−3 to 1×1016 cm−3, and the impurity concentration Na of the well is 1×1017 cm−3 or more, the channel resistance Rch will be 1.8 mΩcm2 or more, which is equal to or more than twice what the channel resistance should be.
Thus, it can be seen that the impurity concentration of the third n-type impurity region 7c is preferably set to 1×1016 cm−3 or more in order to significantly reduce the channel resistance.
In order to reduce the channel resistance, it is preferred that the impurity concentration Next of the third n-type impurity region 7c is as high as possible. However, if the impurity concentration Next is set to 1018 cm−3 or more, a high electric field is applied to the third n-type impurity region 7c when a high voltage is applied to the drain. This is disadvantageous for the OFF characteristics, causing a decrease in the breakdown voltage and an increase in the leak current. In order to decrease the electric field intensity of the gate oxide film at the point P in
Note that while the silicon carbide substrate and the drift layer are of the n-type in the first and second embodiments, effects described above in the first and second embodiments are also realized with a MOSFET that uses a silicon carbide substrate and a drift layer of the p-type and has a structure in which the conductivity type is reversed from the first and second embodiments. The present invention is not limited to MOSFETs, and similar effects can be obtained also when a structure of the present invention is employed with an IGBT.
The present invention is suitably used in power MOSFETs and various control devices and driving devices using power MOSFETs.
Number | Date | Country | Kind |
---|---|---|---|
2008-268508 | Oct 2008 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/JP2009/005249 | 10/8/2009 | WO | 00 | 4/15/2011 |