Semiconductor device and manufacturing method for same, as well as power conversion device

Provided is a vertical MOSFET in which a conduction deterioration phenomenon is prevented during a current return operation and an on-voltage is low during the current return operation. A semiconductor device includes a hole barrier region that is provided between a second-conductivity-type body region and a first-conductivity-type epitaxial layer below a second-conductivity-type body contact region and functions as a potential barrier to a hole which flows from a source electrode to the first-conductivity-type epitaxial layer through the second-conductivity-type body contact region and the second-conductivity-type body region.

TECHNICAL FIELD

The present disclosure relates to a semiconductor device and can be applied to, for example, a power conversion device.

BACKGROUND ART

In a high-voltage inverter device according to the related art which controls the driving of, for example, a motor, a pair of an insulated gate bipolar transistor (IGBT) and a p-intrinsic-n (PIN) diode, which are silicon (Si) elements and connected in inverse parallel, is provided in each of the upper and lower arms. The upper and lower IGBTs are alternately switched to generate an AC current as the output of the inverter device, thereby controlling the motor. During this operation, when the IGBT is turned on, a driving current flows through the IGBT. When the IGBT is turned off, a return current from the motor flows through the diode. In this case, conduction loss and switching loss occur in each of the IGBT and the diode, which causes the power conversion loss of the inverter device. The loss of the Si device at present is substantially equal to a theoretical value which is determined from the physical property value of Si and it is difficult to further reduce the loss.

SiC has a wider forbidden band width than silicon and the breakdown field of SiC is about one order of magnitude greater than that of silicon. Therefore, SiC is expected to be widely used for the next-generation power device. In particular, in a SiC vertical power metal oxide semiconductor field effect transistor (MOSFET), on-resistance is expected to be significantly reduced in a wide breakdown voltage range of several hundreds to several kilovolts, as compared to the silicon element according to the related art. In addition, since the MOSFET is a unipolar element unlike the IGBT, it can perform switching at a high speed. A PN diode which is called a body diode is provided in the structure of the vertical power MOSFET. Therefore, a return current can flow through the body diode, without using an inverse parallel diode. As a result, it is possible to form an inverter device using only the vertical power MOSFET and to reduce the number of components or the size of the inverter device, as compared to the Si inverter according to the related art.

However, in a SiC PN diode, when there is a stacking fault in the element, the stacking fault is grown by the recombination of electrons and holes during the turn-on of the diode. As a result, the lifetime of a hole which is conducted is reduced and the forward on-voltage of the PN diode increases (conduction deterioration occurs).

In contrast, in Patent Document 1, a region into which a recombination center is introduced is provided on a current path immediately after a semiconductor device is turned on. Therefore, a chance of the recombination of electrons and holes due to the stacking fault is reduced and the growth of the stacking fault and conduction deterioration are prevented.

CITATION LIST

Patent Document

Patent Document 1: JP 2009-164440 A

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

However, in the semiconductor device disclosed in Patent Document 1, when there is a stacking fault in the vicinity of the region in which the recombination center has been introduced, the growth of the stacking fault is accelerated. As a result, conduction deterioration is likely to be accelerated.

In general, the region in which the recombination center has been introduced has low crystallinity and has a plurality of carrier trap levels. Therefore, the following various problems are likely to occur: the breakdown voltage is significantly lower than a value which is expected from the physical property value of SiC; the forward current of the PN diode is significantly reduced; a transient response is delayed; and hysteresis occurs.

An object of the disclosure is to provide a semiconductor device which prevents conduction deterioration in a body diode and has good current return characteristics.

Solutions to Problems

The outline of a representative structure of the disclosure will be described in brief as follows.

A semiconductor device includes a hole barrier region that is provided between a second-conductivity-type body region and a first-conductivity-type epitaxial layer below a second-conductivity-type body contact region and functions as a potential barrier to a hole which flows from a source electrode to the first-conductivity-type epitaxial layer through the second-conductivity-type body contact region and the second-conductivity-type body region.

Effects of the Invention

According to the above-mentioned semiconductor device, it is possible to prevent a conduction deterioration phenomenon during a current return operation and to obtain good characteristics in the current return operation.

MODES FOR CARRYING OUT THE INVENTION

The outline of an embodiment will be described in brief as follows.

A semiconductor device (1) includes: a first-conductivity-type semiconductor substrate (10); a first-conductivity-type epitaxial layer (12) that is provided on the first-conductivity-type semiconductor substrate (10); a second-conductivity-type body region (13) that is formed in a predetermined portion of a surface of the first-conductivity-type epitaxial layer (12); a first-conductivity-type source region that is formed in a predetermined portion of a surface of the second-conductivity-type body region (13); a second-conductivity-type body contact region (15) that is provided in a predetermined portion of the surface of the second-conductivity-type body region (13); a source electrode (20) that is formed on the first-conductivity-type source region (14) and the second-conductivity-type body contact region (15); and a hole barrier region (21) that is provided between the second-conductivity-type body region (13) and the first-conductivity-type epitaxial layer (12) below the second-conductivity-type body contact region (15) and functions as a potential barrier to a hole which flows from the source electrode (20) to the first-conductivity-type epitaxial layer (12) through the second-conductivity-type body contact region (15) and the second-conductivity-type body region (13).

According to the above-mentioned semiconductor device, it is possible to suppress a hole current using the hole barrier region during a current return operation. For an electron current during the current return operation, it is possible to suppress the recombination of electrons and holes in the body region and to obtain return current characteristics with a low on-voltage. In addition, it is possible to prevent a conduction deterioration phenomenon during the current return operation of the semiconductor device.

Hereinafter, embodiments and modifications will be described in detail with reference to the drawings. In all of the drawings for describing the embodiments and the modifications, in principle, the same members are denoted by the same reference numerals and the description thereof will not be repeated.

FIG. 1Ais a diagram illustrating the cross-sectional structure of a semiconductor device according to Embodiment 1.FIG. 1Bis a diagram illustrating the planar structure of the semiconductor device according to Embodiment 1.FIG. 1Ais a cross-sectional view taken along the line C-C′ ofFIG. 1B.FIG. 1Bis a top view from which a source electrode20and an interlayer insulating film22illustrated inFIG. 1Aare removed.

A semiconductor device1according to this embodiment includes a drain electrode11which is provided on a lower surface of an n+(first conductivity type) SiC substrate10, an n (first conductivity type) epitaxial layer12which is formed on an upper surface of the n+(first conductivity type) SiC substrate10, and a p-type (second conductivity type) body region13which is selectively formed in the surface of the n−(first conductivity type) epitaxial layer12. In addition, the semiconductor device1includes an n+(first conductivity type) source region14and a p+ (second conductivity type) body contact region15which are selectively provided in a surface portion of the p-type (second conductivity type) body region13and a channel region17which is provided in the surface portion of the body region13so as to be adjacent to the source region14. The semiconductor device1further includes a gate electrode19which is provided above the channel region17, with a gate insulating film18interposed therebetween, and a source electrode20which is formed on the source region14and the body contact region15. The semiconductor device1further includes a hole barrier layer21which is provided below the body contact region15and prevents the flow of holes from the source electrode20to the epitaxial layer12through the body contact region15and the body region13. The semiconductor device1forms a vertical MOSFET. For example, the concentration of the n−epitaxial layer12is in the range of 1015/cm3to 1016/cm3and the concentration of the p-type body region13is in the range of 1017/cm3to 1018/cm3, the concentration of the n+source region14is in the range of 1019/cm3to 1020/cm3, and the concentration of the p-type body contact region15is in the range of 1019/cm3to 1020/cm3.

In this embodiment, the interlayer insulating film22is formed on the gate electrode19and the source electrode20on the interlayer insulating film22is electrically separated from the gate electrode19. The source electrode20is electrically connected to the source region14and the body contact region15through a source contact hole which is formed in the interlayer insulating film22on the source region14and the body contact region15. The semiconductor device1includes a plurality of cells which are periodically arranged in parallel.FIG. 1Aillustrates one cell of the semiconductor device1.

Next, the operation of the semiconductor device1will be described. The semiconductor device1forms the vertical MOSFET (vertical power MOSFET). When the vertical MOSFET is turned on, a positive bias is applied between the gate electrode19and the source electrode20(VGS>0 V). In this case, an inversion layer (channel) including electrons is formed in the channel region17below the gate electrode19and the voltage applied between the source electrode20and the drain electrode11causes an electron, which is a carrier, to flow between the source electrode20and the drain electrode11through the source region14, the channel region17, an n−(first conductivity type) JFET region16which is provided between adjacent body regions13, the n−epitaxial layer12which will be a drift layer, and the SiC substrate10.

During the current return operation, the vertical MOSFET is in an off state (VGS≦0 V) and no channel is formed. FIG.2is a diagram illustrating a current path during the current return operation. During the current return operation, a positive voltage is applied from the drain to the source (VSD>0 V) and a hole (h+) flows from the source electrode20to the epitaxial layer12through the body contact region15and the body region13, as represented by a current path A inFIG. 2. The path A indicates a hole current, a path A′ indicates an electron current, and a path B′ indicates an electron current.FIG. 3is a band diagram along the current paths A and A′ illustrated inFIG. 2when no bias is applied (VSD=0 V). Here, the hole barrier layer21is doped with, for example, n-type (first conductivity type) impurities with a higher concentration than the n−epitaxial layer12and functions as a barrier which prevents the outflow of the hole (h+). Therefore, it is possible to prevent electrical conduction caused by the hole carrier and to prevent conduction deterioration. For example, the concentration of the hole barrier layer21is in the range of 1017/cm3to 1019/cm3.

FIG. 4is a band diagram along the current path B′ illustrated inFIG. 2when no bias is applied (VSD=0 V). When VSD>0 V is satisfied during the current return operation, an electron (e−) flows from the drain electrode11to the source region14through the SiC substrate10and the epitaxial layer12. Here, in the p-type body region13below the path B′, when the concentration of p-type impurities is low or when the path length of a p-type region is short, the electron (e−) which flows from the epitaxial layer12to the body region13is not recombined with the hole (h+) in the p-type body region13and can reach the source region14. When the sufficient amount of current in the path is ensured, it is possible to obtain good current return characteristics with a low on-voltage even though the hole current is suppressed. In addition, since the current in the path is the electron current, the problem of conduction deterioration does not occur.

Modification

FIG. 5is a diagram illustrating the cross-sectional structure of a semiconductor device according to a modification. A semiconductor device1A includes a hole barrier layer21which extends below the path B′.FIG. 6is a band diagram along the current path B′ when no bias is applied (VSD=0 V). When the hole barrier layer21is doped with n-type impurities with a higher concentration than the n−epitaxial layer12, the amount of electron diffusion current which flows to the p-type body region13below the path B′ increases and the amount of electron drift current caused by the electric field also increases. Therefore, it is possible to ensure the electron current. Here, the hole barrier layer is formed below the p-type body region13. However, an upper portion of the hole barrier layer21may extend into the p-type body region13. Since the current in the path is the electron current, the problem of conduction deterioration does not occur.

According to the above-mentioned structure, during the current return operation (VSD>0 V), the flow of the hole current through the path A is prevented by the hole barrier layer21. Therefore, it is possible to prevent conduction deterioration and to prevent the recombination of electrons and holes in the p-type body region13below the path B′. As a result, it is possible to ensure the electron current, that is, to obtain good current return characteristics with a low on-voltage.

Manufacturing Method

Next, a process of manufacturing the semiconductor devices according to this embodiment and the modification will be described. First, the n+SiC substrate10is prepared. The n−epitaxial layer12is formed on the n+SiC substrate10by epitaxial growth. Then, for example, aluminum (Al) ions are implanted into a predetermined region of a surface layer of the n−epitaxial layer12by a photolithography technique to form the p-type body region13. Here, for example, boron (B) other than Al may be implanted as the p-type impurities. Similarly, the n+source region14and the p′ body contact region15are formed in a predetermined region of the surface layer of the p-type body region13by the photolithography and an ion implantation technique. For example, nitrogen (N) or phosphorus (P) is used as n-type ion implantation impurities.

Here, during ion implantation for forming the p+body contact region15, n-type ion implantation with higher energy is added to form the hole barrier layer21below the p+body contact region15. In this case, when ion implantation is performed in an oblique direction, it is possible to form the structure according to the modification illustrated inFIG. 5in which the hole barrier layer21extends in the left-right direction. Alternatively, as another method for forming the structure according to the modification illustrated inFIG. 5, during ion implantation for forming the n+source region14, n-type ion implantation with higher energy may be added. However, it is preferable that the hole barrier layer21illustrated inFIG. 5not extend to an end portion of the p-type body region13close to the JFET region16. The end portion of the p-type body region13is a portion in which the electric field is high due to the concentration of the electric field when the MOSFET is in the off state and a high voltage is maintained. Therefore, when the hole barrier layer21extends to the end portion, the breakdown voltage is reduced.

After the ion implantation process, for example, a heat treatment is performed at a temperature of about 1700° C. to activate the implanted impurities.

Then, for example, an insulating film with a thickness of about a few tens of nanometers is formed on the surface of the epitaxial layer12by a thermal oxidation method or a chemical vapor deposition (CVD) method. Then, for example, polysilicon heavily doped with P or B is deposited on the insulating film by the CVD method and the insulating film and the polysilicon are processed by a photolithography technique and a dry etching technique to form the gate insulating film18and the gate electrode19in a predetermined region. The gate insulating film18is, for example, a SiO2film, a SiON film, an Al2O3film, a HfO2film, or a stacked film thereof.

Then, the interlayer insulating film22is deposited by, for example, the CVD method. Then, a predetermined region of the interlayer insulating film22is processed by photolithography and the dry etching technique to form a source contact hole. In addition, a metal film, such as an Al film, is deposited by, for example, the CVD method, a vapor deposition method, or a sputtering method to form the source electrode20. Similarly, a metal film, such as a titanium (Ti) film, a nickel (Ni) film, a gold (Au) film, a silver (Ag) film, or a stacked film thereof, is deposited on the lower surface of the SiC substrate10to form the drain electrode11.

Finally, a passivation film is deposited and a pad region for applying a bias to each electrode from the outside is formed in the passivation film, which is not illustrated in the drawings. In this way, the semiconductor devices1and1A according to the embodiment and the modification are completed.

In Embodiment 1, the electron which passes through the channel region from the source when the semiconductor device is turned on needs to pass through the JFET region16which is provided in the relatively narrow surface portion of the n−epitaxial layer12between adjacent p-type body regions13. The resistance of the region is relative high, which causes an increase in the resistance of the vertical MOSFET.

When the width of the JFET region (in the left-right direction in the drawings) increases in order to reduce the on-resistance, the width of one cell, that is, the pitch between the cells which are periodically arranged increases. In this case, the number of cells to be arranged in a chip with a given size is reduced, which is equivalent to an increase in the resistance of the semiconductor device. The increase in the cell pitch leads to an increase in resistance during the current return operation.

In this embodiment, an example of a semiconductor device with lower on-resistance in a vertical MOSFET will be described.FIG. 7is a diagram illustrating the cross-sectional structure of the semiconductor device according to Embodiment 2. In this embodiment, components having the same functions as those in Embodiment 1 are denoted by the same reference numerals and the description thereof will not be repeated.

A semiconductor device1B according to this embodiment is characterized in that a gate insulating film18and a gate electrode19are buried in a trench (groove) portion which is formed in an epitaxial layer12, unlike Embodiment 1. Hereinafter, a manufacturing process will be sequentially described.

Similarly to Embodiment 1, an n−epitaxial layer12is formed on an n+SiC substrate10. Then, a p-type body region13, an n+source region14, a p body contact region15, and a hole barrier layer21are formed by ion implantation and a heat treatment is performed to activate impurities.

Then, a predetermined region of a surface portion of the epitaxial layer12is processed by photolithography and a dry etching technique to form a trench region.

Then, similarly to Embodiment 1, for example, an insulating film with a thickness of a few tens of nanometers is formed on the surface of the epitaxial layer12including the surface of the trench region by a thermal oxidation method or a CVD method. Then, polysilicon doped with, for example, P or B is deposited on the insulating film by the CVD method. In this case, it is preferable that the polysilicon of the trench region portion be formed so as to seamlessly cover the bottom and side wall of the trench.

In addition, the insulating film and the polysilicon are processed by the photolithography and the dry etching technique to form a gate insulating film18and a gate electrode19in a predetermined region.

Then, an interlayer insulating film22, a source contact hole, a source electrode20, and a drain electrode11are formed through the same process as that in Embodiment 1. In this way, the semiconductor device1B according to this embodiment is completed.

In this embodiment, a JFET region is not provided between adjacent p-type body regions13. When the vertical MOSFET according to this embodiment is turned on, the p-type body region13which is provided on the side surface of the trench portion, with the gate insulating film18interposed therebetween, is changed to a channel region17by a positive bias applied to the gate electrode19and an inversion layer channel is formed.

Since electrons which pass through the channel directly flow to the n-type epitaxial layer12, it is possible to further reduce on-resistance.

The width of the trench is reduced to reduce the pitch between cells. Therefore, it is possible to reduce resistance during the current return operation.

According to this structure, similarly to Embodiment 1, it is possible to suppress a hole current using the hole barrier layer21and thus to prevent the problem of conduction deterioration.

In Embodiment 1 and Embodiment 2, the hole barrier layer21is provided at the boundary between the body region13and the epitaxial layer12below the body contact region15. When the MOSFET is in an off state and a high voltage is maintained, the intensity of the electric field is high at the interface between the p-type body region13and the n−epitaxial layer12which is the boundary of a PN junction. In Embodiment 1 and Embodiment 2, since the n-type impurity concentration of the hole barrier layer21is higher than that of the n−epitaxial layer12, there is a problem that the intensity of the electric field of the PN junction below the body contact region15is high.

This embodiment is characterized in that a hole barrier layer21is provided in a body region13below a body contact region15.FIG. 8is a diagram illustrating the cross-sectional structure of a semiconductor device according to Embodiment 3. In this embodiment, components having the same functions as those in Embodiments 1 and 2 are denoted by the same reference numerals and the description thereof will not be repeated.

In a semiconductor device1C according to this embodiment, the p-type body region13extends below the hole barrier layer21(p-type body region13′). Therefore, when a high voltage is maintained, the intensity of the electric field is high at the interface between the p-type body region13′ and an n−epitaxial layer12. The intensity of the electric field can be lower than that in Embodiment 1 and Embodiment 2. For holes which flow from a source electrode20through the body contact region15, a hole current can be suppressed by the hole barrier layer21, similarly to Embodiment 1 and Embodiment 2.

Since the body region13below the source region14has the same structure as that in Embodiment 1, it is possible to ensure the sufficient amount of electron current through a path B′.

Next, a manufacturing process will be sequentially described.

Similarly to Embodiment 1, the n−epitaxial layer12is formed on an n+SiC substrate10. Then, the p-type body region13, an n-type source region14, and the p-type body contact region15are formed by ion implantation. Here, when the p-type body contact region15is formed, the hole barrier layer is formed by n-type ion implantation with high energy, similarly to Embodiment 1. In this embodiment, p-type ion implantation with higher energy is added and an additional p-type body region13′ is formed below the hole barrier layer21. In this case, it is preferable that the addition p-type body region13′ not extend below the source region14. When the p-type body region13below the source region14is deeply formed, the amount of electron current is reduced during the current return operation.

Then, a heat treatment is performed to activate impurities and a gate insulating film18and a gate electrode are formed similarly to Embodiment 1. In addition, an interlayer insulating film22, a source contact hole, a source electrode20, and a drain electrode11are formed through the same process as that in Embodiment 1. In this way, the semiconductor device1C according to this embodiment is completed.

FIG. 9is a diagram schematically illustrating the structure of a power conversion device according to Embodiment 4. A power conversion device31includes a three-phase inverter device32using the semiconductor device according to Embodiments 1 to 3 and the modification, a load circuit33, such as a motor, a power supply34, and a capacitor35. InFIG. 9, each of switching elements37u,37v,37w,37x,37y, and37zis formed by the semiconductor device1,1A,1B, or10. The switching elements37u,37v, and37ware provided in an upper arm and the switching elements37x,37y, and37zare provided in a lower arm. The switching elements37uand37xare used for the U-phase, the switching elements37vand37yare used for the V-phase, and the switching elements37wand37zare used for the W-phase.

Gate driving circuits36uand36xdrive the switching elements37uand37x, respectively. Gate driving circuits36vand36ydrive the switching elements37vand37y, respectively. Gate driving circuits36wand36zdrive the switching elements37wand37z, respectively. The DC power supply34and the capacitor35are connected between one end (drain node) N1of each switching element provided in the upper arm and one end (source node) N2of each switching element provided in the lower arm. A voltage (VPP) is applied between the drain node N1and the source node N2. Each gate driving circuit appropriately turns on and off the corresponding switching element to generate three-phase (the U-phase, the V-phase, and the W-phase) AC signals with different phases from VPP which is a DC signal. The load33is appropriately controlled by the three-phase (the U-phase, the V-phase, and the W-phase) AC signals.

A body diode is provided in the structure of the semiconductor device1,1A,1B, or1C and a return current can flow through the body diode, without using an inverse parallel diode (freewheeling diode). Therefore, it is possible to reduce the number of components in the inverter device or the size of the inverter device.

The invention made by present inventors has been described in detail above on the basis of the embodiments, the examples, and the modifications. However, the invention is not limited to the embodiments, the examples, and the modifications and it is needless to say that various modifications and changes of the invention can be made without departing from the scope of the invention.

REFERENCE SINGS LIST