JP2008227414A - Semiconductor device and method of manufacturing same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce JFET resistance value of a semiconductor device having a broad buffer structure. <P>SOLUTION: In the semiconductor device, a N<SP>-</SP>drift layer 1 has at least one portion whose net doping concentration becomes maximum. The net doping concentration of the N<SP>-</SP>drift layer 1 becomes low from a portion that the doping concentration becomes maximum to both P base region 2 and N<SP>+</SP>collector buffer layer 4. Out of surface layers at a first main surface side of the N<SP>-</SP>drift layer 1 having a predetermined depth, a second conductive impurity is more deeply introduced in a region not in contact with a gate insulation film 6 than in a second conductive base region. Further, out of surface layers having a predetermined depth at a first main surface side of the N<SP>-</SP>drift layer 1, the net doping concentration in contact with the gate insulation film 6 is higher than that of the region not in contact with the gate insulation film 6. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a semiconductor device and a manufacturing method thereof, and more particularly to an IGBT (insulated gate bipolar transistor) having not only high speed and low loss but also soft switching characteristics and a manufacturing method thereof.

  As a power semiconductor device, there are a diode, IGBT, and the like of a withstand voltage class such as 600V, 1200V, or 1700V. Recently, the characteristics of these devices have been improved. Power semiconductor devices are used in power conversion devices such as highly efficient and power-saving converter-inverters, and are indispensable for control of rotary motors and servo motors.

Such power control devices are required to have characteristics such as low loss and power saving, high speed and high efficiency, and environmental friendliness, that is, no adverse effects on the surroundings. In response to such a demand, a broad buffer structure has been proposed in the diode. The broad buffer structure, N ^- average concentration distribution of the drift layer has a peak (maximum value) substantially near the middle of the same layer, and towards the emitter and collector direction, so as to decrease with an inclination structure (For example, see Patent Document 1 below).

  In the diode of the broad buffer structure, high-speed operation (for example, carrier frequency: 20 kHz or more) that is difficult with the conventional technique for reducing the emitter injection efficiency and controlling the lifetime distribution (for example, see Patent Document 2 below). Soft recovery characteristics and oscillation suppression effect can be realized. Patent Document 1 discloses the following two methods for manufacturing such a broad buffer diode.

The first method is a method in which a region having a phosphorus concentration higher than the initial phosphorus concentration of the semiconductor substrate is formed by an epitaxial growth method in a deep region in the bulk, that is, in a region 30 to 60 μm or deeper than the surface of the semiconductor chip. It is. The second method is a method in which proton ions (H ⁺ ) are irradiated to a FZ (floating zone) bulk wafer and heat treatment is performed, whereby protons are converted into donors in the vicinity of the range Rp inside the bulk. Since the bulk wafer is less expensive than the epitaxial wafer, the second method is less expensive than the first method.

In addition to Patent Document 1, various methods for forming a high-concentration N ⁺ layer using a proton donor phenomenon caused by proton irradiation and heat treatment have been proposed (for example, Patent Document 3 and Patent below). Reference 4). In addition, Patent Document 4 discloses a method of forming an N ⁺ layer using a thermal donor using oxygen. In addition, when it is necessary to avoid proton donor formation, there is a proposal to use helium instead of proton (see, for example, Patent Document 5 below).

  Further, as a method for realizing a broad buffer structure at a low cost, a method for obtaining a high concentration region inside a bulk as a net doping concentration by compensating a donor (phosphorus) concentration of a semiconductor substrate with an acceptor element has been proposed (for example, , See Patent Document 6 below). Further, a method is known in which defects are formed in a silicon substrate by proton irradiation and the residual defects are adjusted by heat treatment to locally reduce the lifetime (see, for example, Patent Document 5 below).

  Further, as a method for manufacturing an IGBT with low cost and low electrical loss, a conventional semiconductor substrate (for example, a silicon wafer) is thinned by grinding or the like, and then an element is ion-implanted from the ground surface side at a predetermined concentration to perform heat treatment. The method of performing is known (for example, refer to Patent Document 7 below). In recent years, the development and manufacture of devices by such low-cost methods are becoming mainstream.

  In addition to the field stop type IGBT disclosed in Patent Document 7 described above, the soft punch-through type IGBT disclosed in Patent Document 8 below, as an IGBT having improved both low loss characteristics and soft recovery characteristics, The intermediate high concentration layer insertion type IGBT disclosed in the following Patent Document 9 and Patent Document 10 is known.

JP 2003-318812 A JP-A-8-148699 Japanese Patent Laid-Open No. 9-260639 JP 2001-156299 A Japanese Patent Laid-Open No. 2003-249662 JP-A-2005-64429 Japanese translation of PCT publication No. 2002-52085 JP 2000-195870 A JP-A-3-44969 JP 2004-193212 A

However, when the IGBT broad buffer layer is formed by the compensation effect by the acceptor element as in Patent Document 6, the net doping concentration of the portion immediately below the MOS gate in the N ⁻ drift layer is lowered by the compensation action. For this reason, there exists a problem that JFET (Junction Field-Effect Transistor) resistance of IGBT will become high.

FIG. 12 is a diagram showing a configuration of an IGBT having a broad buffer structure according to the prior art. 12, a P base region 1402 and an N ⁺ emitter region 1403 are formed on one main surface side of the N ⁻ drift layer 1401 as shown in a sectional view 1400 of the semiconductor device. An N ⁺ buffer layer 1404 is formed on the other main surface side of the N ⁻ drift layer 1401. A P ⁺ collector layer 1405 is formed on the surface of the N ⁺ buffer layer 1404. Further, a MOS gate structure portion including a gate insulating film 1406 and a gate electrode 1407 is formed on the surface of the N ⁻ drift layer 1401 on the P base region 1402 side (hereinafter, the MOS gate structure portion is referred to as a gate insulating film). And the gate electrode are collectively expressed). A collector electrode 1408 is formed on the surface of the P ⁺ collector layer 1405.

The JFET resistance is generated in the N ⁻ drift layer 1401 immediately below the center in the width direction of the MOS gate structure (region indicated by the symbol J). In the region D on the surface of the N ⁻ drift layer 1401 on the P base region 1402 side, an acceptor element such as aluminum or gallium is uniformly diffused in the width direction. For this reason, the net doping concentration immediately below the MOS gate structure where the JFET resistance is generated is low.

The resistance value of the JFET resistance of the IGBT having the conventional broad buffer structure (hereinafter referred to as the JFET resistance value) can be obtained as follows. Under the conditions shown in FIG. 10 of Patent Document 6, the net doping concentration on the surface on the MOS gate side is 5 × 10 ¹³ atoms / cm ³ . In this case, the JFET resistance value R _JFET is expressed by the following formula (1) (see Reference Document 1 below). In the following formula (1), ρ _D is the specific resistance of the JFET resistor, W _bi is the built-in depletion layer width, L _g is the gate width (width of the MOS gate structure), and x _j is the diffusion of impurities in the P base region. Depth 2m is the width of the source opening.
(Reference 1: BJ Baliga, “Power Semiconductor Devices”, PWS Publishing, 1996, p371.

If the gate width L _g is 20 μm, the doping concentration of the P base region is 2 × 10 ¹⁷ atoms / cm ³ , the impurity diffusion depth x _j in the P base region is 3 μm, and the width 2 m of the source opening is 10 μm, From equation (1), the JFET resistance value R _JFET is 2584 Ωcm ² . Thus, the JFET resistance value is a very large value.

  In order to reduce the JFET resistance value, a method such as JFET implantation is known in which a donor element such as phosphorus is ion-implanted into the JFET resistance portion to reduce the JFET resistance value. However, a method such as JFET implantation increases the number of steps and increases the manufacturing cost of the device.

Further, if the net doping concentration of the entire N ⁻ drift layer is increased in order to reduce the JFET resistance value, there is a problem that the breakdown voltage of the device is reduced. This phenomenon is verified by introducing the distance index W _bm . The distance index W _bm is obtained by using the depletion layer width W ₁ required for the withstand voltage BV as a guideline (= index) of the thickness of the N ⁻ drift layer. The distance index W _bm is expressed by the following formula (2). In the following formula (2), BV is a device breakdown voltage, and N _dm is an average doping concentration of the N ⁻ drift layer.
W _bm = 2BV / (4010N _dm ^1/8 ) (2)

The above equation (2) is derived as follows. First, from the above reference 1 (page 74), when the doping concentration of the N ⁻ drift layer is Nd, the critical electric field strength E _c is expressed by the following formula (3). Here, the average doping concentration N _dm of the N ⁻ drift layer corresponds to N _d .
E _c = 4010N _d ^1/8 (3)

The element withstand voltage BV by the parallel plate approximation is expressed by the following formula (4), where W ₁ is the depletion layer width when the maximum electric field strength in the element reaches the critical electric field strength E _{c due} to an increase in applied voltage.
BV = E _c W _1/2 (4)

By replacing the depletion layer width W ₁ of the above equation (4) with the distance index W _bm and substituting the above equation (3) into E _c of the above equation (4), the above equation (2) is obtained.

FIG. 13 shows the relationship between the distance index W _bm and the element withstand voltage BV. FIG. 13 is a chart showing an example of the relationship between the distance index W _bm and the element breakdown voltage. Table 1500 and Table 1510 in FIG. 13 show the rated breakdown voltage, average specific resistance, average net doping concentration N _dm , critical electric field strength Ec, and distance index W _bm corresponding to each element breakdown voltage BV.

The typical rated withstand voltage of power semiconductor elements (IGBT, FWD, etc.) that are generally used is set to be about 10% lower than the element withstand voltage BV, and safety is taken into consideration. The average net doping concentration N _dm indicates a typical net doping concentration for obtaining each device breakdown voltage BV. The average net doping concentration of devices that are actually sold in mass is slightly different from the values shown in Table 1500, but is close enough as an order. In Table 1500, the distance index W _bm of a device with a rated withstand voltage of 600 V is about 57 μm, and the distance index W _bm of a device with a rated voltage of 1200 V is about 126 μm. Further, for example, as shown in Table 1510, the average net doping concentration N _dm may be designed to be slightly lower than that in Table 1500 so as to provide a margin for the breakdown voltage.

In order to set the device breakdown voltage as shown in Table 1500 and Table 1510 of FIG. 13, the average net doping concentration of the N ⁻ drift layer needs to be as shown in the same table. However, in the broad buffer layer, the peak concentration (the net doping concentration at the apex in the mountain distribution) is higher than the average net doping concentration, and the net doping concentrations on the front side and the back side of the broad buffer layer (the bottom portion in the mountain distribution) Is lower than the average net doping concentration.

For this reason, the net doping concentration in the portion directly under the MOS gate on the surface is relatively low, and the JFET resistance is high. If the concentration just below the MOS gate is increased in order to reduce the JFET resistance, it is inevitably necessary to increase the peak concentration, so that a sufficient depletion layer width cannot be ensured and the breakdown voltage is lowered. A similar problem occurs when the thickness of the N ⁻ drift layer is reduced.

  Further, although the device can be formed with a trench gate structure that does not include a JFET resistance portion, the number of steps increases as compared with the planar gate structure, and the manufacturing cost of the device increases. In addition, in a device having a trench gate structure, it is difficult to maintain the yield rate because the gate breakdown voltage is affected by the quality of the trench sidewall.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor device having a broad buffer structure in which the resistance value of a JFET resistor is reduced in order to eliminate the above-described problems caused by the prior art. It is another object of the present invention to provide a semiconductor device manufacturing method capable of manufacturing a semiconductor device having a broad buffer structure in which a resistance value of a JFET resistor is reduced at low cost and with good controllability using an FZ bulk wafer. .

  In order to solve the above-described problems and achieve the object, a semiconductor device according to the invention of claim 1 is selectively formed on a first conductive type semiconductor substrate and a first main surface side of the first conductive type semiconductor substrate. Second conductive type base region, a first conductive type source region selectively formed on the surface of the second conductive type base region, and the first conductive type semiconductor substrate of the second conductive type base region And a gate insulating film in contact with a portion sandwiched between the first conductive type source region and a gate electrode on the gate insulating film, the first conductive type source region, and the second conductive type base region An emitter electrode in contact with the first conductive type semiconductor substrate, a second conductive type collector layer formed on the second main surface side of the first conductive type semiconductor substrate, and a collector electrode in contact with the second conductive type collector layer, The first conductive type semiconductor substrate There are at least one portion where the impurity concentration of the first conductivity type semiconductor substrate becomes maximum, and the second conductivity type base region and the region where the impurity concentration of the first conductivity type semiconductor substrate becomes maximum In the semiconductor device that is lowered toward both of the second conductivity type collector layers, the gate insulating film of the surface layer of a predetermined depth on the first main surface side in the first conductivity type semiconductor substrate, A second conductivity type impurity is introduced deeper than the second conductivity type base region into the non-contact region, and the net doping concentration in the region in contact with the gate insulating film is the net doping concentration in the region not in contact with the gate insulating film. It is characterized by being higher than the doping concentration.

A semiconductor device according to a second aspect of the present invention is the semiconductor device according to the first aspect, wherein the second conductive type impurity introduced into the first conductive type semiconductor substrate is on the surface of the first conductive type semiconductor substrate. A ratio β between the concentration and the impurity concentration of the first conductivity type semiconductor substrate satisfies the following expression.

  According to a third aspect of the present invention, in the semiconductor device according to the first or second aspect, the first conductive semiconductor substrate is an N-type silicon substrate, and the second conductive impurity is made of boron. Is a P-type impurity element having a large diffusion coefficient.

  According to a fourth aspect of the present invention, in the semiconductor device according to the third aspect, the second conductivity type impurity is any one of aluminum, gallium, platinum, and zinc.

  According to the present invention, the net doping concentration of the region in contact with the gate insulating film, that is, the region where the JFET resistance is generated, in the surface layer of the first conductivity type semiconductor substrate having the predetermined depth on the MOS gate structure side is set as the gate insulating film. The JFET resistance of the semiconductor device can be reduced by making it higher than the net doping concentration in the region not in contact with the semiconductor device.

  According to a fifth aspect of the present invention, there is provided a method of manufacturing a semiconductor device according to the first aspect of the present invention, in manufacturing the semiconductor device according to the first aspect, the step of forming the MOS gate structure on the first conductivity type semiconductor substrate, Second conductivity type impurities are introduced from the surface of the conductive semiconductor substrate where the MOS gate structure is formed and the surface where the MOS gate structure is not formed, and the net doping concentration of a part of the first conductive semiconductor substrate is reduced. And a step of lowering the initial net doping concentration of the first conductivity type semiconductor substrate.

  According to a sixth aspect of the present invention, there is provided a method of manufacturing a semiconductor device according to the first aspect of the present invention, in the step of forming the MOS gate structure on the first conductivity type semiconductor substrate, A second conductivity type impurity is introduced from a surface of the conductivity type semiconductor substrate on which the MOS gate structure is formed, and the gate insulating film of the surface layer on the first main surface side in the first conductivity type semiconductor substrate And a step of lowering a net doping concentration in a region not in contact with a region lower than a net doping concentration in a region in contact with the gate insulating film.

  According to a seventh aspect of the present invention, there is provided a semiconductor device manufacturing method according to the fifth or sixth aspect, wherein the first conductive semiconductor substrate is an N-type silicon substrate, and the first conductive semiconductor substrate. The second conductivity type impurity introduced into the surface on which the MOS gate structure is formed is a P-type impurity element having a diffusion coefficient larger than that of boron.

  According to an eighth aspect of the present invention, there is provided a method for manufacturing a semiconductor device according to the seventh aspect of the present invention, wherein the second conductive material is introduced into a surface of the first conductive semiconductor substrate on which the MOS gate structure is formed. The type impurity is aluminum or gallium.

According to a ninth aspect of the present invention, there is provided a method for manufacturing a semiconductor device according to the eighth aspect, wherein the aluminum or the gallium is ion-implanted at a dose of 1 × 10 ¹⁴ atoms / cm ³ or less. And

  According to a tenth aspect of the present invention, there is provided a method for manufacturing a semiconductor device according to the ninth aspect, wherein after the ion implantation of the aluminum or the gallium, heat treatment is performed at a temperature of 900 ° C. to 1200 ° C. And

  According to an eleventh aspect of the present invention, there is provided a method for manufacturing a semiconductor device according to the ninth aspect, wherein the second conductivity introduced into the surface of the first conductivity type semiconductor substrate on which the MOS gate structure is formed. The type impurity is platinum or zinc.

A method of manufacturing a semiconductor device according to a twelfth aspect of the invention is characterized in that, in the invention of the eleventh aspect, the platinum or the zinc is ion-implanted at a dose of 1 × 10 ¹⁵ atoms / cm ³ or less. And

  According to a thirteenth aspect of the present invention, there is provided a method for manufacturing a semiconductor device according to the twelfth aspect of the present invention, wherein after the ion implantation of the platinum or the zinc, heat treatment is performed at a temperature of 400 ° C. or higher and 1000 ° C. or lower. And

  According to the present invention, the net doping concentration of the region in contact with the MOS gate structure side, that is, the region where the JFET resistance is generated, of the surface layer of the first conductivity type semiconductor substrate having the predetermined depth on the MOS gate structure side is determined as the MOS gate. A semiconductor device in which the JFET resistance is reduced by making it higher than the net doping concentration in a region not in contact with the structure side can be manufactured.

  According to the semiconductor device of the present invention, a semiconductor device having a broad buffer structure in which the resistance value of the JFET resistor is reduced can be obtained. Further, according to the method for manufacturing a semiconductor device according to the present invention, a semiconductor device having a broad buffer structure in which the resistance value of the JFET resistance is reduced can be manufactured at low cost and with good controllability using an FZ bulk wafer. .

  Exemplary embodiments of a semiconductor device and a method for manufacturing the same according to the present invention will be explained below in detail with reference to the accompanying drawings. In the present specification and the accompanying drawings, it means that electrons or holes are majority carriers in the layers and regions with N or P, respectively. Further, + and − attached to N and P mean that the impurity concentration is relatively high or low, respectively. Note that, in the following description of the embodiments and the accompanying drawings, the same reference numerals are given to the same components, and duplicate descriptions are omitted.

(Embodiment 1)
FIG. 1 is a diagram illustrating the configuration and characteristics of the semiconductor device according to the first embodiment. In the following description, the direction indicated by the line segment XX ′ in the cross-sectional view 100 of FIG. 1 is the vertical direction of the semiconductor device, and the direction indicated by the line segment YY ′ is the width direction of the semiconductor device.

As shown in a cross-sectional view 100 of the semiconductor device in FIG. 1, in the semiconductor device according to the first embodiment, a P base region 2 and an N ⁺ emitter region 3 are formed on one main surface side of the N ⁻ drift layer 1. ing. An N ⁺ buffer layer 4 is formed on the other main surface side of the N ⁻ drift layer 1. A P ⁺ collector layer 5 is formed on the surface of the N ⁺ buffer layer 4. On the surface of the N ⁻ drift layer 1 on the P base region 2 side, a MOS gate structure portion and an emitter electrode 8 composed of a gate insulating film 6 and a gate electrode 7 are formed. A collector electrode 9 is formed on the surface of the P ⁺ collector layer 5.

The distance from the emitter electrode 8 in FIG. 1 - as shown in the characteristic diagram 110 of net doping concentration (log), N ^- net doping concentration of the drift layer 1, N ^- peak substantially near the middle of the drift layer 1 And decrease toward the P base region 2 and the N ⁺ buffer layer 4 with an inclination. That is, the semiconductor device of the first embodiment has a broad buffer structure. The net doping concentrations of P base region 2, N ⁺ buffer layer 4 and P ⁺ collector layer 5 are all higher than the net doping concentration of N ⁻ drift layer 1.

Net doping concentration of the P base region 2 is 2 × 10 ¹⁷ atoms / cm ³ at the interface with the emitter electrode 8, N ^- becomes lower toward the drift layer 1, N ^- at the interface between the drift layer 1, 5 × 10 ¹³ atoms / cm ³ lower. N ^- net doping concentration of the drift layer 1, P at the interface between the base region 2, 5 × but 10 ¹³ less than atoms / cm ^3, P base in the vicinity of the interface between the region ^{2 5 × 10 13 atoms / cm} 3 It becomes.

Then, the net doping concentration at the peak at about the middle of the N ⁻ drift layer 1 is 1.5 × 10 ¹⁴ atoms / cm ³ . The net doping concentration of the N ⁻ drift layer 1 at the interface with the N ⁺ buffer layer 4 and in the vicinity thereof is 5 × 10 ¹³ atoms / cm ³ .

The net doping concentration of the N ⁺ buffer layer 4 is 5 × 10 ¹³ atoms / cm ³ at the interface with the N ⁻ drift layer 1 and increases toward the P ⁺ collector layer 5. The net doping concentration of the N ⁺ buffer layer 4 is higher than the net doping concentration of the peak of the N ⁻ drift layer 1 in the vicinity of the interface with the P ⁺ collector layer 5, which is 1.5 × 10 ¹⁴ atoms / cm ³ . At the interface with the P ⁺ collector layer 5, it is lower than 5 × 10 ¹³ atoms / cm ³ .

The net doping concentration of the P ⁺ collector layer 5 is lower than 5 × 10 ¹³ atoms / cm ^{3 at} the interface with the N ⁺ buffer layer 4, but increases toward the collector electrode 9, and 1 at the interface with the collector electrode 9. × 10 ¹⁸ atoms / cm ³

The donor concentration of the entire substrate of the semiconductor device is 1.5 × 10 ¹⁴ atoms / cm ³ . Aluminum (Al) or gallium (Ga) is diffused as an acceptor from the interface between the emitter electrode 8 and the P base region 2 to almost the center of the N ⁻ drift layer 1 (near the net doping concentration peak). Further, zinc (Zn) or platinum (Pt) is diffused as an acceptor from the P ⁺ collector layer 5 side to almost the center of the N ⁻ drift layer 1. This acceptor concentration is lower than the donor concentration of the entire substrate at each position.

Further, as shown in the characteristic diagram 110 in FIG. 1, the distance from the emitter electrode 8 to the interface between the P base region 2 and the N ⁻ drift layer 1 is 3 μm. The distance from the emitter electrode 8 to the interface between the P ⁺ collector layer 5 and the collector electrode 9 is 120 μm. The distance from the interface between the N ⁺ buffer layer 4 and the P ⁺ collector layer 5 to the interface between the P ⁺ collector layer 5 and the collector electrode 9, that is, the thickness of the P ⁺ collector layer 5 is 0.5 μm.

  Next, a manufacturing process of the semiconductor device shown in FIG. 1 will be described. 2 and 3 are views showing a manufacturing process of the semiconductor device shown in FIG. First, an N-type FZ wafer 10 having a specific resistance of, for example, 30 Ωcm and a mirror finished surface is prepared as a semiconductor substrate. An oxide film having a thickness of 8000 mm is formed on one main surface of the FZ wafer 10.

Next, as shown in a sectional view 200 of FIG. 2, patterning is performed on the main surface of the FZ wafer 10 on which the oxide film is formed, and an edge termination portion is opened by wet etching. Next, boron is ion-implanted from the main surface of the FZ wafer 10 on which the oxide film is formed. The acceleration voltage at that time is, for example, 100 keV, and the dose amount is, for example, 1 × 10 ¹⁵ atoms / cm ² . Then, for example, heat treatment is performed at 1150 ° C. for 200 minutes to diffuse boron and form a guard ring structure (not shown). Subsequently, the active portion is opened by wet etching, leaving the oxide film 21 in the edge termination portion. Next, an insulating film having a thickness of 650 mm is grown. Subsequently, a polysilicon film having a thickness of 1.0 μm is formed. Then, the insulating film and the polysilicon film are patterned to form the gate insulating film 6 and the gate electrode 7 (see the sectional view 210).

Next, as shown in a sectional view 210 of FIG. 2, from the main surface of the FZ wafer 10 on which the gate electrode 7 is formed (hereinafter, this surface is referred to as the front surface of the FZ wafer 10), the acceptor element As an ion implantation, aluminum (Al) or gallium (Ga) is ion-implanted. The acceleration voltage at that time is, for example, 100 keV, and the dose amount is, for example, 1 × 10 ¹⁴ atoms / cm ² . At this time, aluminum or gallium is injected from the opening not covered with the gate electrode 7 and the gate insulating film 6.

The reason why the dose of aluminum or gallium is set to 1 × 10 ¹⁴ atoms / cm ² is to allow diffusion of implanted ions into the atmosphere due to outward diffusion during the heat treatment. The donor concentration necessary for forming the broad buffer layer by compensating the donor concentration of the FZ wafer 10 is about 3 × 10 ¹¹ / cm ² . On the other hand, due to outward diffusion during the heat treatment, the amount of ions remaining in the wafer after the heat treatment becomes 0.1% to 1% of the dose. If the dose of aluminum or gallium is 1 × 10 ¹⁴ atoms / cm ² or less, ions of about 1 × 10 ¹¹ to 1 × 10 ¹² atoms / cm ² remain in the wafer. Therefore, 3 × 10 ¹¹ / cm ² necessary for forming the broad buffer layer can be secured.

Then, for example, heat treatment is performed at 1200 ° C. for 10 hours to diffuse the implanted ions. By this heat treatment, ions implanted into the opening are diffused into the FZ wafer 10. The dotted line in the cross-sectional view 210 schematically shows ions diffused into the FZ wafer 10. For example, the diffusion coefficient of aluminum is about 1 × 10 ⁻¹¹ (cm ² / s) at 1150 ° C. (see Reference 2 below). Therefore, when the implanted ions are aluminum, the implanted ions are diffused by about 30 μm by the heat treatment.

Note that the temperature and time of the heat treatment after ion implantation of aluminum or gallium can be set as appropriate in the range of 1000 ° C. to 1200 ° C. and 1 hour to 100 hours. This is because the structure formed on the surface of the FZ wafer 10 is only the thermal oxide film and the polysilicon film which is the gate electrode 7 and can be processed at a high temperature.
(Reference 2) D.J. Fisher, “Diffusion in Silicon 10 Years of Research”, [online], Society Publications (SCITEC PUBLICATIONS), Internet <URL: http: // www. mse. ufl. edu / ￣spear / semi_properties / data / 5029. pdf>

Next, as shown in a sectional view 220 of FIG. 2, boron (B) is ion-implanted from the front surface of the FZ wafer 10. The acceleration voltage at that time is, for example, 100 keV, and the dose amount is, for example, 1 × 10 ¹⁴ atoms / cm ² . Then, for example, heat treatment is performed at 1150 ° C. for 120 minutes to diffuse the implanted ions. The boron ions form the P base region 2 (see the sectional view 230). Further, by this heat treatment, the aluminum or gallium diffused earlier diffuses further into the FZ wafer 10.

The diffusion coefficient of aluminum or gallium is larger than that of boron. For this reason, the diffusion depth after the heat treatment after boron implantation is deeper for aluminum or gallium than for boron. Aluminum or gallium diffused deeper than boron reaches the N ⁻ drift layer 1. The N ^- by aluminum or gallium having reached the drift layer 1, N ^- it is possible to control the net doping concentration of the drift layer 1.

Next, patterning is performed on the front surface of the FZ wafer 10, and arsenic (As) is ion-implanted from the front surface of the FZ wafer 10 as shown in a sectional view 230 of FIG. The acceleration voltage at that time is, for example, 100 keV, and the dose amount is, for example, 5 × 10 ¹⁵ atoms / cm ² . Then, for example, heat treatment is performed at 1000 ° C. for 30 minutes to diffuse the implanted ions. The arsenic ions form the N ⁺ emitter region 3 (see cross-sectional view 300). Thereafter, a BPSG film is formed on the front surface of the FZ wafer 10 and patterned to form an interlayer insulating film in the gate portion.

  Subsequently, as shown in a cross-sectional view 300 of FIG. 3, the main surface of the FZ wafer 10 on the side where the gate electrode 7 is not formed (hereinafter, this surface is referred to as the back surface of the FZ wafer 10) is ground. The thickness is 160 μm. Subsequently, wet grinding is performed on the ground surface of the FZ wafer 10 to make the wafer thickness 140 μm.

Next, as shown in a cross-sectional view 310 of FIG. 3, selenium (Se) or phosphorus (P) is ion-implanted from the back surface of the FZ wafer 10. The acceleration voltage at that time is, for example, 100 keV, and the dose amount is, for example, 1 × 10 ¹³ atoms / cm ² . Then, for example, heat treatment is performed at 1000 ° C. for 1 hour, and the implanted ions are diffused to form the N ⁺ buffer layer 4.

Further, as shown in the cross-sectional view 320 of FIG. 3, zinc (Zn) or platinum (Pt) is ion-implanted from the back surface of the FZ wafer 10. The acceleration voltage at that time is, for example, 100 keV, and the dose amount is, for example, 1 × 10 ¹⁵ atoms / cm ² . Then, for example, heat treatment is performed at 800 ° C. for 1 hour to diffuse the implanted ions. For example, the diffusion coefficient of zinc is about 1 × 10 ⁻¹¹ (cm ² / s) at 800 ° C. (see Reference 2 above). Therefore, when the implanted ions are zinc, the broad buffer layer is formed by diffusing the implanted ions by about 30 μm by the heat treatment.

Subsequently, boron (B) is ion-implanted from the back surface of the FZ wafer 10. The acceleration voltage at that time is, for example, 45 keV, and the dose amount is, for example, 1 × 10 ¹⁴ atoms / cm ² . Then, for example, heat treatment is performed at 450 ° C. for 1 hour to diffuse the implanted ions. Further, instead of heat treatment, ions may be activated by irradiation with a laser beam such as a YAG laser by a double pulse method. At this time, the energy density of the laser beam is, for example, 3 J / cm ² . The delay time of the double pulse is, for example, 300 nsec. The activated boron ions form the P ⁺ collector layer 5 (see the sectional view 330).

  Here, the double pulse method is a method of continuously irradiating a plurality of pulse lasers by shifting the irradiation timing by a predetermined delay time from a plurality of laser irradiation apparatuses for each laser light irradiation area. The double pulse method is described in detail in Japanese Patent Application Laid-Open No. 2005-223301.

Then, 5 μm of Al—Si is sputtered to form a film and patterned and etched to form the emitter electrode 8 as shown in the cross-sectional view 330 of FIG. Finally, titanium, nickel and gold are vapor-deposited on the back surface of the FZ wafer 10 to form the collector electrode 9. The region between the P base region 2 and the N ⁺ buffer layer 4 of the FZ wafer 10 becomes the N ⁻ drift layer 1. Thereafter, the wafer is diced along the scribe lines to form individual devices. A characteristic diagram 340 in FIG. 3 is a net doping concentration profile corresponding to the semiconductor device in the cross-sectional view 330.

Note that zinc (Zn) or platinum (Pt) may be implanted as an acceptor element instead of aluminum or gallium. In this case, the gate insulating film 6, the gate electrode 7, and the like are formed on the FZ wafer 10 in the same procedure as the sectional view 200 of FIG. Next, boron is ion-implanted into the FZ wafer 10 by the same procedure as that of the cross-sectional view 220 of FIG. 2, heat-treated and diffused to form the P base region 2 (see the cross-sectional view 230). Subsequently, as shown in a sectional view 230 of FIG. 2, arsenic (As) is ion-implanted into the FZ wafer 10 and diffused to form an N ⁺ emitter region 3 (see the sectional view 300). Thereafter, a BPSG film is formed on the FZ wafer 10 and patterned to form an interlayer insulating film in the gate portion.

Then, zinc or platinum is ion-implanted from the front surface of the FZ wafer 10. The dose of zinc or platinum is about 1 × 10 ¹⁵ atoms / cm ² or less. The reason is that the electrical activity of zinc or platinum is about 0.01 to 0.1, so that the donor concentration necessary to form the broad buffer layer is compensated by compensating the donor concentration of the wafer. This is because a dose amount 10 times that of aluminum or gallium is required.

  Subsequently, for example, heat treatment is performed at 800 ° C. for 30 minutes to diffuse the implanted ions. Note that the temperature and time of the heat treatment after ion implantation of zinc or platinum are desirably in the range of 400 ° C. to 1000 ° C., 0.5 hour to 100 hours. This is because zinc and platinum have high diffusion coefficients and can be diffused by heat treatment at about 400 ° C. Further, since a thermal oxide film, a gate polysilicon film, and an interlayer oxide film are formed on the surface of the FZ wafer 10, it is desirable to perform the treatment at 1000 ° C. or less for 0.5 to 100 hours.

  Then, similarly to the description of the sectional views 300 to 330 in FIG. 3, grinding and wet etching are performed on the back surface of the FZ wafer 10 to reduce the thickness of the FZ wafer 10 (see the sectional view 300 in FIG. 3). Then, selenium or phosphorus is ion-implanted from the ground surface of the FZ wafer 10 and heat treatment is performed (see a cross-sectional view 310 in FIG. 3).

  Further, zinc or platinum is ion-implanted from the ground surface of the FZ wafer 10 and heat treatment is performed (see a cross-sectional view 320 in FIG. 3). Subsequently, boron (B) is ion-implanted from the ground surface of the FZ wafer 10 to perform heat treatment. Finally, the emitter electrode 8 and the collector electrode 9 are formed (see the cross-sectional view 330 in FIG. 3). Through the above process, the semiconductor device shown in FIG. 1 can be formed.

Note that heat treatment is not performed immediately after ion implantation of zinc or platinum from the surface, and after second ion implantation of zinc or platinum from the back surface (see cross-sectional view 320 in FIG. 3), 800 ° C. for 1 hour. The surface-side implanted ions and the back-side implanted ions may be diffused simultaneously by performing a heat treatment. This is because platinum and zinc have a smaller diffusion coefficient than aluminum and gallium, and therefore can be processed at a temperature lower than their thermal history after forming the P base region 2 and the N ⁺ emitter region 3. is there. In this way, the threshold value of the MOS gate can be easily managed by simultaneously diffusing the implanted ions on the front surface side and the implanted ions on the back surface side.

  Instead of zinc or platinum, an element having a higher diffusion coefficient than boron, such as silver (Ag) or beryllium (Be), may be used as the acceptor element.

  FIG. 4 is a diagram schematically showing the diffusion direction of the acceptor element (aluminum or gallium, zinc or platinum) in the FZ wafer. As shown in FIG. 4, the acceptor element implanted into the surface of the opening S where the MOS gate structure formed of the gate insulating film 6 and the gate electrode 7 is not formed diffuses mainly from the opening S in the vertical direction. . Therefore, the acceptor element is not diffused in the portion directly under the MOS gate structure (shaded portion in FIG. 4), or the amount of diffusion is smaller than that in the portion directly under the opening S.

For this reason, the N-type net doping concentration in the portion immediately below the MOS gate structure is relatively higher than the N-type net doping concentration on the surface of the opening S. For example, under the conditions shown in FIG. 2 and FIG. 3, the N-type net doping concentration in the portion immediately below the MOS gate structure is about 2.3 × 10 ¹⁴ / cm ³ . As a result, the JFET resistance value of the semiconductor device shown in FIG. 4 is 296 Ωcm ² , which can be reduced to about 1/10 compared with the JFET resistance value of the semiconductor device having the conventional structure.

  FIG. 5 is a graph showing the trade-off characteristics of turn-off loss and on-voltage of the semiconductor device shown in FIG. 1 and the semiconductor device according to the prior art. In FIG. 5, the vertical axis represents the turn-off loss (mJ) of the semiconductor device, and the horizontal axis represents the on-voltage (V) of the semiconductor device. In FIG. 5, white circles (◯) indicate characteristic values of the semiconductor device (the present invention) shown in FIG. 1. Also, the white triangle (Δ) indicates the characteristic value of the semiconductor device (conventional example) according to the prior art. As shown in FIG. 5, in the semiconductor device shown in FIG. 1, the on-voltage is reduced by about 0.1 V as compared with the semiconductor device according to the prior art, and the trade-off characteristics between the on-voltage and the turn-off loss are improved. You can see that

  Next, various parameters for controlling net doping in a semiconductor device having a broad buffer structure will be described. FIG. 6 is a graph showing the relationship between various parameters in the broad buffer structure. In FIG. 6, the vertical axis represents the doping concentration (log), and the horizontal axis represents the distance (μm) from one main surface (for example, the front surface) of the wafer. Hereinafter, details of the equation shown in FIG. 6 will be described. In the following description, it is assumed that the acceptor diffusion distribution by ion implantation and heat treatment follows the same Gaussian function f (x) on both the front side and the back side of the wafer.

The distance from one main surface (for example, the front surface) of the wafer to the other main surface (for example, the back surface), that is, the thickness that characterizes the diffusion depth in the Gaussian function (W ₀ (μm)) (Hereinafter referred to as “diffusion length”) is L ₀ (μm), the depth direction distance from the front surface is x (μm), and the surface concentration of the compensation acceptor is C ₀ (atoms / cm ³ ). The net distribution f (x) of the compensation acceptor diffused from both sides is expressed by the following formula (5).

From the above equation (5), the net doping concentration distribution g (x) of the broad buffer structure obtained by compensation is expressed by the following equation (6). In the following formula (6), N ₀ is the doping concentration of the FZ wafer (hereinafter referred to as “substrate concentration”). Also, g (x)> 0 is always satisfied.
g (x) = N ₀ −f (x) (6)

Next, a parameter α relating W ₀ and L ₀ is substituted. Here, α is defined by the following formula (7). Using α, the net distribution f (x) of the compensation acceptor is expressed by the following equation (8).

When g (x = 0) = g (x = W ₀ ) is G (α), G (α) is expressed by the following equation (9). From the following equation (9), G (α) is a value obtained by subtracting the compensation acceptor concentration f (0) on the front surface or the back surface from the substrate concentration N ₀ . Also, G (α)> 0 is always satisfied.

Next, a parameter β relating the surface concentration C _{0 of} the front or back compensation acceptor and the substrate concentration N ₀ is introduced as follows. β is defined by the following formula (10). Using β, G (α) is defined by the following formula (11).

Here, the following equation (12) is derived from G (α)> 0 and N ₀ > 0.

The above formula (12) means that the ratio of the surface concentration C ₀ of the compensation acceptor to the substrate concentration N ₀ is at least less than 1. This is equivalent to the fact that the net compensation acceptor concentration f (0) on one side (for example, the front side) due to diffusion of compensation acceptors from both sides is less than N ₀ .

Here, the net doping concentration (N ₀ −f (0)) of the broad buffer structure on the front surface (or the back surface) is preferably 8% to 40% of N ₀ . When G (α) is larger than 40% of N ₀ , the electric field strength in the depletion layer is greatly reduced, and a sufficient depletion layer width cannot be secured with the critical electric field strength. Therefore, if G (α) is larger than 40% of N ₀ , the breakdown voltage of the device cannot be secured. This condition is expressed by the following formula (13).

The above formula (13) is represented by the following formula (15) using h (α) defined by the following formula (14). That is, it is desirable that the surface concentration C ₀ of the compensation acceptor satisfies the relationship of the following formula (15) with respect to the concentration N ₀ of the FZ wafer substrate.

Here, in general, the diffusion distribution of the compensation acceptor from the front surface and the back surface is not exactly equal. However, a desired broad buffer distribution can be obtained by setting the surface concentration C ₀ of the front and back compensation acceptors within a range that satisfies the relationship of the above equation (15).

  Also, the above formula (15) can be applied to the case where the donor is compensated by the acceptor only on one side of the wafer. The case where the acceptor is compensated only on one side of the wafer is, for example, a method for forming a broad buffer structure as follows. First, a broad buffer distribution gradient is formed by epitaxial growth on the back side of a high concentration CZ-N type wafer or a high concentration CZ-P type wafer. After the peak concentration, the epitaxial growth is performed with the concentration fixed. Thereafter, the compensation acceptor is diffused from the front surface of the wafer to form a broad buffer distribution on the surface side. In this way, a broad buffer structure can be made even if only one side of the wafer is compensated by the acceptor.

Next, the relationship between the parameters α and β is shown in FIG. 7 from the above equations (12) and (15). FIG. 7 is a graph showing the relationship between parameters α and β. In FIG. 7, the vertical axis indicates the value of β, and the horizontal axis indicates the value of α. Α and β when G (α) = 0 are set to α ₀ and β ₀ , respectively. In FIG. 7, the thick solid lines indicate the values of α ₀ and β ₀ , that is, the values of the following formula (16). The value of β takes a value below the curve represented by the following formula (16) (β ≦ β ₀ ). The dotted line shows a value when the β = 0.92β _0. Also, the thin solid line indicates the value when the β = 0.6β _0.

If the wafer thickness W ₀ is 3 times or more of the diffusion length L ₀ (W ₀ > 3L ₀ , α> 3), the value that β can take is 1 or less. On the other hand, when the wafer thickness W ₀ is gradually made smaller than three times the diffusion length L ₀ (W ₀ ≦ 3L ₀ , α ≦ 3), the value that β can take is smaller than 1, W ₀ ≦ 0.1 L ₀ , That is, when α ≦ 0.1, the value is approximately 0.5 or less. Here, when α ≦ 0.1, the specific meaning that the value that β can take is 0.5 or less will be described. In the case of W ₀ ≦ 0.1L ₀ (α ≦ 0.1), the diffusion distribution of the compensation acceptor from the front surface and the back surface is substantially uniform at the surface concentration C ₀ . Therefore, N ₀ needs to be 2C ₀ or more which is the sum of the surface concentrations on the front side and the back side (N ₀ ≧ 2C ₀ ), so that β can have a value of 0.5 or less.

Further, from the above equation (15), the value of β is recommended to be a region between 0.92β ₀ indicated by the dotted line in FIG. 7 and 0.6β ₀ indicated by the thin solid line (shaded portion in FIG. 7). Value.

Next, the relationship (α dependency) between the net doping concentration g (x) and the diffusion length L ₀ of the compensation acceptor will be verified. FIG. 8 is a graph showing the relationship between the net doping concentration and the diffusion length L ₀ of the compensation acceptor when the wafer thickness W _O is 120 μm. FIG. 8 shows a net doping concentration distribution of a semiconductor device having a vertical broad buffer distribution. For example, the semiconductor device is moved from the emitter electrode 8 toward the collector electrode 9 as indicated by a line XX ′ in FIG. 2 shows the net doping concentration distribution along the cross section divided. (For simplicity, we will ignore the N ⁺ buffer layer and the P ⁺ collector layer.)

In the graphs 801 to 805 in FIG. 8, the vertical axis is the net doping (broad buffer) concentration (atoms / cm ³ ), and the horizontal axis is from one main surface of the wafer (for example, the surface on the emitter electrode 8 side in FIG. 1). Distance (μm). The graphs 801 to 805 each have a substrate concentration N ₀ of 4.0 × 10 ¹⁴ atoms / cm ³ and a compensation acceptor surface concentration C ₀ of 3.4 × 10 ¹⁴ atoms / cm ³ , and the compensation acceptor diffusion length. L is ₀ only allowed to change graph obtained.

A graph 801 shows the net doping concentration distribution when the diffusion length L ₀ of the compensation acceptor is 79.37 μm. Under the conditions of the graph 801, α = 1.512, β = 0.85, and h (α) = 0.9077. Therefore, β is not in the range of 0.6h (α) = 0.5446 to 0.92h (α) = 0.8351, which is the condition of the above formula (15).

Graph 802 shows the net doping concentration distribution when the diffusion length L ₀ of the compensation acceptor is 77.46 μm. Under the conditions of the graph 802, α = 1.549, β = 0.85, and h (α) = 0.9168. Therefore, β is not in the range of 0.6h (α) = 0.5501 to 0.92h (α) = 0.8435, which is the condition of the above formula (15).

A graph 803 is a net doping concentration distribution when the diffusion length L ₀ of the compensation acceptor is 67.08 μm. Under the conditions of the graph 803, α = 1.789, β = 0.85, and h (α) = 0.9608. Therefore, it is in the range of 0.6h (α) = 0.5765 to 0.92h (α) = 0.8840, which is the condition of the above formula (15).

A graph 804 is a net doping concentration distribution when the diffusion length L ₀ of the compensation acceptor is 59.16 μm. Under the conditions of the graph 804, α = 2.03, β = 0.85, and h (α) = 0.9839. Therefore, β is in the range of 0.6h (α) = 0.5904 to 0.92h (α) = 0.9052, which is the condition of the above formula (15).

A graph 805 shows a net doping concentration distribution when the diffusion length L ₀ of the compensation acceptor is 44.72 μm. Under the conditions of the graph 805, α = 2.683, β = 0.85, and h (α) = 0.993. Therefore, it is within the range of 0.6h (α) = 0.5996 to 0.92h (α) = 0.9193, which is the condition of the above formula (15).

Thus, in the case of the conditions of graphs 801 and 802 (L ₀ = 79.37 μm, 77.46 μm), the value of β is not in the range of the above formula (15), so the net doping concentration is extremely low. It has become. Therefore, the semiconductor device created under the conditions of the graphs 801 and 802 is not practical. On the other hand, in the case of the conditions of graphs 803 to 805 (L ₀ = 67.08 μm, 59.16 μm, 44.72 μm), the value of β is in the range of the above formula (15), so that the net doping concentration is a desired value. Broad buffer distribution.

Next, the relationship (β dependence) between the net doping concentration g (x) and the surface concentration C ₀ of the compensation acceptor will be verified. FIG. 9 is a graph showing the relationship between the net doping concentration and the surface concentration C ₀ of the compensation acceptor when the diffusion length L ₀ of the compensation acceptor is 59.16 μm. FIG. 9 shows the net doping concentration distribution of a semiconductor device having a broad buffer distribution in the vertical direction, as in FIG. 8. For example, as shown by the line XX ′ in FIG. 2 shows a net doping concentration distribution along a cross-section divided from 1 to the collector electrode 9. (For simplicity, we will ignore the N ⁺ buffer layer and the P ⁺ collector layer.)

In the graphs 901 to 905 in FIG. 9, the vertical axis represents the net doping (broad buffer) concentration (atoms / cm 3), and the horizontal axis represents the distance from one main surface of the wafer (for example, the surface on the emitter electrode 8 side in FIG. 1). (Μm). Graphs 901 to 905 are obtained by changing the substrate concentration N ₀ to be 4.0 × 10 ¹⁴ atoms / cm ³ , the compensation acceptor diffusion length L ₀ to be constant at 59.15 μm, and changing only the compensation acceptor surface concentration C _0. Is a graph.

A graph 901 is a net doping concentration distribution when the surface concentration C ₀ of the compensation acceptor is 3.9 × 10 ¹⁴ atoms / cm ³ . Under the conditions of the graph 901, α = 2.828, β = 0.975, and h (α) = 0.9839. Therefore, β is not in the range of 0.6h (α) = 0.5904 to 0.92h (α) = 0.9052, which is the condition of the above formula (15).

A graph 902 is a net doping concentration distribution when the surface concentration C ₀ of the compensation acceptor is 3.6 × 10 ¹⁴ atoms / cm ³ . Under the condition of the graph 902, β = 0.900. Α and h (α) are the same as the conditions in the graph 901. Therefore, β is within the range of the condition of the above formula (15).

A graph 903 is a net doping concentration distribution when the surface concentration C ₀ of the compensation acceptor is 3.4 × 10 ¹⁴ atoms / cm ³ . Under the condition of the graph 903, β = 0.85. Α and h (α) are the same as the conditions in the graph 901. Therefore, β is within the range of the condition of the above formula (15).

A graph 904 is a net doping concentration distribution when the surface concentration C ₀ of the compensation acceptor is 3.0 × 10 ¹⁴ atoms / cm ³ . Under the condition of the graph 904, β = 0.750. Α and h (α) are the same as the conditions in the graph 901. Therefore, β is within the range of the condition of the above formula (15).

A graph 905 is a net doping concentration distribution when the surface concentration C ₀ of the compensation acceptor is 2.0 × 10 ¹⁴ atoms / cm ³ . Under the condition of the graph 905, β = 0.500. Α and h (α) are the same as the conditions in the graph 901. Therefore, β is not within the range of the condition of the above formula (15).

Under the condition of the graph 901 (C ₀ = 3.9 × 10 ¹⁴ atoms / cm ³ ), β = 0.975, which is very close to the substrate concentration N ₀ . For this reason, the surface concentration of the net doping concentration is lower than 1.0 × 10 ¹³ atoms / cm ³ . Such distribution is effective for ensuring the breakdown voltage of the device. However, it is difficult to form a concentration difference between the surface concentration C ₀ of the compensation acceptor and the substrate concentration N _{0 of} this condition by ion implantation and diffusion. Therefore, a semiconductor device formed under the conditions of the graph 901 is not practical.

Further, under the condition of graph 905 (C ₀ = 2.0 × 10 ¹⁴ atoms / cm ³ ), the net doping concentration shows a broad buffer distribution, but the peak concentration is 2.6 × 10 ¹⁴ atoms / cm ^3. The minimum density is 2.0 × 10 ¹⁴ atoms / cm ³ , and the difference between the peak density and the minimum density is small. A device formed under the conditions of the graph 905 has a withstand voltage of about 700 V, which is less than 1200 V. Therefore, a semiconductor device formed under the conditions of the graph 905 is not practical.

On the other hand, in the case of the conditions of graphs 902 to 904 (C ₀ = 3.6 × 10 ¹⁴ atoms / cm ³ , 3.4 × 10 ¹⁴ atoms / cm ³ , 3.0 × 10 ¹⁴ atoms / cm ³ ), Since the value of β is in the range of the above formula (15), the net doping concentration has a desired broad buffer distribution.

Next, the relationship (gate width dependency) between the net doping concentration and the width of the MOS gate structure (hereinafter referred to as “gate width”) will be verified. FIG. 10 is a graph showing the relationship between the net doping concentration and the gate width. FIG. 10 shows a net doping concentration distribution along the cross section divided in the direction along the line ZZ ′ in the cross sectional view of the semiconductor device shown in FIG. 11 (for simplicity, an N ⁺ buffer layer, P ⁺ Ignore the collector layer.)

  FIG. 11 schematically shows a structure of the semiconductor device shown in FIG. Each code | symbol of FIG. 11 has shown the same structure as each code | symbol of FIG. As shown in FIG. 11, the line segment Z-Z ′ is directly under the MOS gate structure and connects between adjacent P base regions 2. Therefore, the vicinity of the center in the horizontal direction of each graph indicates the net doping concentration of the portion immediately below the MOS gate structure.

In the graphs 1001 to 1004 in FIG. 10, the vertical axis represents the net doping (broad buffer) concentration (atoms / cm ³ ), and the horizontal axis represents the distance in the width direction (μm). In each graph of FIG. 10, parameters other than the gate width are the same as those of the graph 801 of FIG. That is, in each graph of FIG. 10, the net doping concentration in the vertical direction is assumed to have a distribution shown in the graph 801 of FIG. Also, consider replacing the W ₀ in the above description to the gate width. Further, when the acceptor is diffused using the gate as a diffusion window, the diffusion depth in the lateral direction is assumed to be 0.7 times the diffusion depth in the vertical direction (see Reference 3 below).
(Reference 3) SM Sze, “Semiconductor Device”, 1996

  A graph 1001 is a net doping concentration distribution when the gate width is 120 μm. Under the conditions of the graph 1001, α = 3.098, β = 0.850, and h (α) = 0.9999. Therefore, β is in the range of 0.6h (α) = 0.6000, which is the condition of the above formula (15), to 0.92h (α) = 0.9199.

  A graph 1002 shows the net doping concentration distribution when the gate width is 100 μm. Under the conditions of the graph 1002, α = 2.582, β = 0.85, and h (α) = 0.9987. Therefore, β is in the range of 0.6h (α) = 0.5992 to 0.92h (α) = 0.9188, which is the condition of the above formula (15).

  A graph 1003 shows the net doping concentration distribution when the gate width is 80 μm. Under the conditions of the graph 1003, α = 2.066, β = 0.85, and h (α) = 0.9862. Therefore, β is in the range of 0.6h (α) = 0.5917 to 0.92h (α) = 0.0703, which is the condition of the above formula (15).

  A graph 1004 shows the net doping concentration distribution when the gate width is 60 μm. Under the conditions of the graph 1004, α = 1.549, β = 0.85, and h (α) = 0.9168. Therefore, it is not in the range of 0.6h (α) = 0.5501 to 0.92h (α) = 0.8435, which is the condition of the above formula (15).

  As described above, under the conditions of the graphs 1001 to 1003 (gate width of 80 μm or more), the value of β is in the range of the above formula (15), and the doping concentration becomes high immediately below the center in the width direction of the MOS gate structure. ing. Therefore, the JFET resistance value can be reduced under the conditions of the graphs 1001 to 1003. On the other hand, under the condition of the graph 1004 (gate width 60 μm), the value of β is not in the range of the above formula (15), and the doping concentration is very low just below the center in the width direction of the MOS gate structure. This concentration is lower than the lowest net doping concentration in the vertical direction (see graph 801 in FIG. 8). Therefore, the JFET resistance becomes extremely high under the condition of the graph 1004.

  In the above-described embodiment, the present invention is applied to the IGBT. However, the same effect can be obtained even when the present invention is applied to a MOSFET that is the same MOS gate.

  As described above, according to the semiconductor device and the manufacturing method thereof according to the present invention, the JFET resistance value of the semiconductor device having the broad buffer structure can be easily reduced with a small number of steps. Therefore, it is possible to obtain a semiconductor device having a turn-off time and turn-off loss that are greatly reduced as compared with the semiconductor device according to the prior art and having smooth and soft switching characteristics.

  As described above, the semiconductor device and the manufacturing method thereof according to the present invention are useful for power semiconductor devices, and are particularly suitable for IGBT modules and IPMs (intelligent power modules) with low electrical loss and radiated electromagnetic noise. .

1 is a diagram illustrating a configuration and characteristics of a semiconductor device according to a first embodiment; It is a figure which shows the manufacturing process of the semiconductor device shown in FIG. It is a figure which shows the manufacturing process of the semiconductor device shown in FIG. It is the figure which showed typically the diffusion direction of the acceptor element in a FZ wafer. 2 is a graph showing trade-off characteristics of turn-off loss and on-voltage of the semiconductor device shown in FIG. 1 and a semiconductor device according to the prior art. It is a graph which shows the relationship of various parameters in a broad buffer structure. It is a graph which shows the relationship between parameter (alpha) and (beta). It is a graph showing the relationship between the diffusion length L ₀ of the net doping concentration and the compensation acceptors. Is a graph showing the relationship between the surface concentration C ₀ of the net doping concentration and the compensation acceptors. It is a graph which shows the relationship between a net doping concentration and gate width. FIG. 2 is a diagram schematically showing the structure of the semiconductor device shown in FIG. 1. It is a figure which shows the structure of IGBT of the broad buffer structure by a prior art. It is a graph which shows an example of the relationship between distance parameter _| index _Wbm and element breakdown voltage.

Explanation of symbols

1 N ⁻ drift layer 2 P base region 3 N ⁺ emitter region 4 N ⁺ collector buffer layer 5 P collector layer 6 gate insulating film 7 gate electrode 8 emitter electrode 9 collector electrode

Claims (13)

A first conductivity type semiconductor substrate, a second conductivity type base region selectively formed on the first main surface side of the first conductivity type semiconductor substrate, and selectively formed on the surface of the second conductivity type base region A first conductive type source region, a gate insulating film in contact with a portion of the second conductive type base region sandwiched between the first conductive type semiconductor substrate and the first conductive type source region, and on the gate insulating film Formed on the second main surface side of the first conductivity type semiconductor substrate, a MOS gate structure comprising a plurality of gate electrodes, an emitter electrode in contact with the first conductivity type source region and the second conductivity type base region. A location where the impurity concentration of the first conductivity type semiconductor substrate is maximized in the first conductivity type semiconductor substrate. There is at least one place And an impurity concentration of said first conductivity type semiconductor substrate, a semiconductor device has become lower toward the portion which becomes the maximum in both of the second conductivity type base region and the second conductivity type collector layer,
Of the surface layer having a predetermined depth on the first main surface side in the first conductivity type semiconductor substrate, a second conductivity type impurity is present in a region not in contact with the gate insulating film from the second conductivity type base region. Deeply introduced,
2. A semiconductor device according to claim 1, wherein a net doping concentration in a region in contact with the gate insulating film is higher than a net doping concentration in a region not in contact with the gate insulating film. The ratio β between the concentration of the second conductivity type impurity introduced into the first conductivity type semiconductor substrate on the surface of the first conductivity type semiconductor substrate and the impurity concentration of the first conductivity type semiconductor substrate is expressed by the following equation: The semiconductor device according to claim 1, wherein:

  The first conductive semiconductor substrate is an N-type silicon substrate, and the second conductive impurity is a P-type impurity element having a diffusion coefficient larger than that of boron. Semiconductor device.   The semiconductor device according to claim 3, wherein the second conductivity type impurity is any one of aluminum, gallium, platinum, and zinc. In manufacturing the semiconductor device according to claim 1,
Forming the MOS gate structure on the first conductive semiconductor substrate;
A second conductivity type impurity is introduced from a surface of the first conductivity type semiconductor substrate on which the MOS gate structure is formed and a surface on which the MOS gate structure is not formed, and a part of the net of the first conductivity type semiconductor substrate is introduced. Lowering the doping concentration below the initial net doping concentration of the first conductivity type semiconductor substrate;
A method for manufacturing a semiconductor device, comprising: In manufacturing the semiconductor device according to claim 1,
Forming the MOS gate structure on the first conductive semiconductor substrate;
A second conductivity type impurity is introduced from a surface of the first conductivity type semiconductor substrate on which the MOS gate structure is formed, and the gate of the surface layer on the first main surface side in the first conductivity type semiconductor substrate is the gate. A step of lowering a net doping concentration of a region not in contact with the insulating film to be lower than a net doping concentration of a region in contact with the gate insulating film;
A method for manufacturing a semiconductor device, comprising:   The first conductivity type semiconductor substrate is an N type silicon substrate, and the second conductivity type impurity introduced into the surface of the first conductivity type semiconductor substrate where the MOS gate structure is formed has a diffusion coefficient higher than that of boron. The method of manufacturing a semiconductor device according to claim 5, wherein the P-type impurity element is large.   8. The method of manufacturing a semiconductor device according to claim 7, wherein the second conductivity type impurity introduced into the surface of the first conductivity type semiconductor substrate on which the MOS gate structure is formed is aluminum or gallium. . The method for manufacturing a semiconductor device according to claim 8, wherein the aluminum or the gallium is ion-implanted at a dose of 1 × 10 ¹⁴ atoms / cm ³ or less.   The method for manufacturing a semiconductor device according to claim 9, wherein a heat treatment is performed at a temperature of 900 ° C. to 1200 ° C. after the ion implantation of the aluminum or the gallium.   8. The method of manufacturing a semiconductor device according to claim 7, wherein the second conductivity type impurity introduced into the surface of the first conductivity type semiconductor substrate on which the MOS gate structure is formed is platinum or zinc. . The method for manufacturing a semiconductor device according to claim 11, wherein the platinum or the zinc is ion-implanted at a dose of 1 × 10 ¹⁵ atoms / cm ³ or less.   The method for manufacturing a semiconductor device according to claim 12, wherein after the ion implantation of the platinum or the zinc, heat treatment is performed at a temperature of 400 ° C. or higher and 1000 ° C. or lower.

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