Adjusting the charge carrier lifetime in a bipolar semiconductor device

Disclosed are a method and a semiconductor device. The method includes implanting recombination center atoms via a first surface into a semiconductor body, and causing the implanted recombination center atoms to diffuse in the semiconductor body in a first diffusion process.

TECHNICAL FIELD

This disclosure in general relates to adjusting the charge carrier lifetime, and more particularly to adjusting the minority charge carrier lifetime in a bipolar power semiconductor device.

BACKGROUND

A bipolar power semiconductor device such as, for example, a power diode, a power IGBT, or a power thyristor, includes a first emitter region of a first conductivity type (doping type), a second emitter region of a second conductivity type, and a base region (often referred to as drift region) of the first conductivity type. Usually, the base region has a lower doping concentration than each of the first and second emitter regions.

A bipolar power semiconductor device can be operated in two different operation states, namely a conducting state (on-state), and a blocking state (off-state). In the conducting state, the first emitter region injects charge carriers of the first conductivity type into the base region, and the second emitter region injects charge carriers of the second conductivity type into the base region. These charge carriers injected into the base region by the first and second emitters form a charge carrier plasma in the base region.

When the bipolar power semiconductor device switches from the conducting state into the blocking state these charge carriers are removed from the base region. Losses that occur in the transition phase from the conducting state to the blocking state are dependent on how many charge carriers are present in the base region before the semiconductor device starts to switch from the conducting state to the blocking state, whereas the higher the amount of charge carriers the higher the losses. Basically, the number of charge carriers can be adjusted by adjusting the charge carrier lifetime, in particular the minority charge carrier lifetime, which is the average time it takes for a minority charge carrier to recombine. The shorter the minority charge carrier lifetime, that is the faster minority charge carriers recombine, the lower is the amount of charge carriers in the base region at the time of switching from the conducting state to the blocking state. However, conduction losses, which are losses that occur in the bipolar power semiconductor device in the conducting state, increase as the charge carrier lifetime decreases.

When the bipolar power semiconductor device switches from the conducting state to the blocking state a depletion region expands in the base region beginning at a pn junction between the base region and the second emitter region. Through this, charge carriers forming the charge carrier plasma are removed from the base region; this is known as reverse recovery. During reverse recovery there is a reverse recovery current flowing between the first and second emitter region. Such reverse recovery current is caused by the removal of charge carriers from the base region. This current finally drops to zero as the charge carriers have been removed or recombined. A slope of this reverse recovery current as it tends to zero defines the softness of the component. The steeper the slope, the less “soft” is the reverse recovery behaviour (switching behaviour) of the semiconductor device. However, a soft behaviour is desirable, because steep slopes may cause voltage overshoots in parasitic inductances connected to the semiconductor device and/or may cause oscillations or ringing in a circuit in which the semiconductor device is employed.

A soft reverse recovery behaviour can be obtained by having a “charge carrier reservoir” in those regions of the base region that are depleted towards the end of the switching process, wherein this charge carrier reservoir feeds the reverse recovery current towards the end of the switching process so as to soften a decrease of the reverse recovery current to zero. Such a “charge carrier reservoir” can be obtained by having a high charge carrier lifetime in those regions of the base region that are depleted towards the end of the reverse recovery process.

There is therefore a need to suitably adjust the charge carrier lifetime in a bipolar semiconductor device in order to have low switching losses and a soft switching behaviour.

SUMMARY

One embodiment relates to a method. The method includes implanting recombination center atoms via a first surface into a semiconductor body, and causing the implanted recombination center atoms to diffuse in the semiconductor body in a first diffusion process.

Another embodiment relates to a semiconductor device. The semiconductor device includes a semiconductor body having a first surface and a second surface, recombination centers in a region between the first surface and the second surface, wherein a profile of the recombination centers in a vertical direction of the semiconductor body comprises a minimum spaced apart from each of the first and second surfaces and at least one maximum. A ratio between the at least one maximum and the minimum is less than 200.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings. The drawings form a part of the description and by way of illustration show specific embodiments in which the invention may be practiced. It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.

FIGS. 1A and 1Bschematically illustrate one embodiment of a method for producing recombination centers in a semiconductor body100in order to adjust the charge carrier lifetime. In particular, the method relates to adjusting the minority charge carrier lifetime in the semiconductor body100. The semiconductor body100includes a first surface101and a second surface102opposite the first surface101.FIGS. 1Aand1B show vertical cross sectional views of a section of the semiconductor body100during different method steps of the method.

The semiconductor body100may include a conventional semiconductor material such as, for example, silicon (Si), silicon carbide (SiC), gallium nitride (GaN), gallium arsenide (GaAs), or the like. AlthoughFIGS. 1A and 1Bonly show one section of one semiconductor body100, the process steps explained with reference toFIGS. 1A and 1B, as well as process steps explained with reference to other figures herein below, can be applied at once to a plurality of semiconductor bodies that are part of a semiconductor wafer. That is, these process steps can be applied to a semiconductor wafer which includes a plurality of semiconductor bodies, wherein the semiconductor wafer can be subdivided into the plurality of semiconductor bodies (dies) at the end of the manufacturing process. According to another embodiment, one wafer includes (is comprised of) only one semiconductor body.

The semiconductor body may include a basic doping such as, for example, an n-type basic doping. For example, a doping concentration of this a basic doping is between 1E12 cm−3and 5E15 cm−3, in particular between 1E13 cm−3and 1E14 cm−3. Dopant atoms that cause the basic doping are, e.g., phosphorous (P) atoms.

Referring toFIG. 1A, the method includes implanting recombination center atoms via the first surface101into the semiconductor body100. Implanting the recombination center atoms may include implanting the recombination centers all over the first surface101. According to another embodiment explained with reference toFIG. 15herein below implanting the recombination center atoms includes the use of an implantation mask that covers sections of the first surface101so as to prevent recombination center atoms from being implanted into these sections. According to one embodiment, implanting the recombination center includes directly implanting the recombination centers into the first surface101. According to another embodiment, the recombination center atoms are implanted through a scattering layer200(illustrated in dashed lines). The scattering layer can be formed on the first surface101(as shown) or can be distant from the first surface101(not shown). According to one embodiment, the scattering layer200includes an oxide such as, for example, silicon oxide (SiO2). A thickness of the scattering layer200may be between 10 nanometers (nm) and 50 nanometers (nm).

According to one embodiment, the recombination center atoms include noble metal atoms, such as, for example, at least one of platinum (Pt) atoms, gold (Au) atoms and palladium (Pd) atoms. How deep the recombination center atoms are implanted into the semiconductor body100is dependent on an implantation energy. According to one embodiment the implantation energy is selected from between 10 keV and 200 keV, in particular between 120 keV and 180 keV. InFIG. 1A, the dashed line labeled with reference character110illustrates the end of range of the implantation. That is, the dashed line illustrates how deep the recombination center atoms are implanted into the semiconductor body100from the first surface101. The distance between the end of range 110 and the first surface101is dependent on the implantation energy and increases as the implantation energy increases. According to one embodiment, the implantation dose is between 1E11 cm−2and 1E14 cm−2in particular between 5E11 cm−2and 5E13 cm−2.

Referring toFIG. 1B, the method further includes a first diffusion process which causes the implanted recombination center atoms to diffuse in the semiconductor body100. In this diffusion process the recombination center atoms diffuse beyond the end of range in the direction of the second surface102but also in the direction of the first surface101. Further, in the diffusion process, the recombination center atoms are incorporated into the crystal lattice on substitutional sites by occupying vacancies of the (monocrystalline) semiconductor body100such that the recombination center atoms may act as recombination centers for charge carriers. This is explained in further detail below.

The distribution of the recombination center atoms after the first diffusion process is dependent on the process parameters such as the implantation dose, the diffusion temperature and the duration of the first diffusion process. According to one embodiment, the diffusion temperature is between 650° C. and 950° C., and the duration of the first diffusion process is between 1 hour and 2 hours. The diffusion temperature can be substantially constant during the first diffusion process. According to another embodiment, the diffusion temperature varies during the first diffusion process.

FIG. 2shows two examples of the distribution of the recombination center atoms in the semiconductor body100after the first diffusion process.FIG. 2shows the distribution of the recombination center atoms over the vertical direction x of the semiconductor body100. Referring toFIG. 2, the distribution of recombination center atoms is basically U-shaped. That is, after the first diffusion process there is a first maximum of the recombination center atom concentration at the first surface101(which corresponds to the vertical position0inFIG. 2), and a second maximum of the recombination center atoms at the second surface102(which corresponds to the vertical position d inFIG. 2), and there is a minimum distant to the first and second surfaces101,102. The recombination center atom concentration in the first maximum at the first surface101and the second maximum at the second surface102can be substantially the same.

FIG. 2shows the distribution of active recombination center atoms in the semiconductor body100. “Active recombination center atoms” are those recombination center atoms that are incorporated in the crystal lattice of the semiconductor body100such that the recombination center atoms may act as recombination centers for charge carriers in the semiconductor body100. How many of the implanted recombination center atoms are activated is mainly dependent on the temperature in the diffusion process. This is explained with reference to two examples. In each of these examples platinum (Pt) atoms are implanted into a silicon (Si) semiconductor body with a thickness d of 500 micrometers (μm). The implantation dose DPtis 5E12 cm−2in both examples. In the two examples, the diffusion parameters in the diffusion process are as follows.

That is, the two examples are only different in the diffusion temperature while the other parameters such as duration of the diffusion process and implantation dose are identical.

The distribution of the recombination center atoms (Pt atoms) obtained in the first example (Example 1) is represented by the distribution curve201shown inFIG. 2, and the distribution of recombination center atoms obtained in the second example (Example 2) is represented by curve302shown inFIG. 2. In the first example, the maximum concentration of activated recombination center atoms in the region of the first and second surfaces101,102is higher than in the second example. The maximum concentration NE1in the first example is about 1E14 cm−3, and the maximum concentration NE2in the second example is about 5E14 cm−3. These maximum concentrations NE1, NE2of the recombination center atoms at the first and the second surface101,102will be referred to as surface concentrations in the following.

FIG. 2shows the distribution of the recombination center atoms on a logarithmic scale. As can be seen fromFIG. 2, the amount of activated recombination center atoms in the second example (see curve302) is significantly higher than in the first example (see curve301). At the lower diffusion temperature (800° C.) in the first example only about 13% of the implanted recombination center atoms are activated, while at the higher diffusion temperature (900° C.) in the second example substantially all (100%) of the implanted recombination center atoms are activated. Thus, the diffusion temperature significantly influences the distribution of the activated recombination center atoms in the method explained with reference toFIGS. 1A and 1B.

Referring toFIG. 2, the diffusion temperature not only influences the amount of recombination center atoms that are activated, but also influences the profile of the distribution. At the higher temperature (see curve302inFIG. 2) the U-shaped profile is shallower. That is, as a result of the higher diffusion temperature a ratio between the surface concentration and the minimum concentration is smaller than in the first example at the lower diffusion temperature. That is, NE2/NM2<NE1/NM1, where NM2is the minimum recombination center concentration in the second example, and NM1is the minimum recombination center concentration in the first example.

In the second example, the surface concentration is below the so-called solubility limit. The “solubility limit” is dependent on the temperature and defines the maximum amount of recombination center atoms that can be activated when employing a conventional process in which recombination center atom are diffused into the semiconductor body from a layer on the surface of the semiconductor body. As will be explained further below, the method explained with reference toFIGS. 1A-1Bmakes it possible to achieve a concentration of recombination centers that is even higher than the solubility limit.

FIG. 3schematically illustrates the solubility limit of platinum (Pt) in silicon (Si) over the temperature. Referring toFIG. 3the solubility increases as the diffusion temperature increases. In case of platinum (as illustrated inFIG. 3) the solubility increases from about 1.0 E13 at 700° C. to about 5.0E16 at 1.000° C. Referring toFIG. 3, the solubility limit at 800° C. is about 1.0E14 cm−3, and at 900° C. is about 2.0E15 cm−3. Thus, in the second example explained before, the surface concentration is below the solubility limit.

In a conventional process for introducing recombination center atoms into a semiconductor body, a metal-semiconductor alloy such as, for example, a platinum silicide is formed on one surface of a semiconductor body, and metal atoms (platinum atoms) diffuse from the alloy into the semiconductor body. In this conventional process, the surface concentration corresponds to the solubility limit. This, however, may cause problems in view of obtaining a soft switching behavior of a semiconductor device that is based on a semiconductor body with those relatively high surface concentrations of recombination center atoms. This explained in further detail herein below. Thus, based on the method explained with reference toFIGS. 1A-1Ba lower surface concentration of recombination center atoms and a shallower U-profile of the recombination center atom distribution can be obtained.

Referring toFIG. 2, the surface concentrations at the first and second surfaces101,102are substantially the same. That is, recombination center atoms that are implanted through the first surface101, in the first diffusion process, diffuse to the first surface101and the second surface102. Thus, the distribution of the recombination center atoms is widely independent of whether the recombination center atoms are implanted through the first surface101or the second surface102. Referring to one embodiment shown inFIG. 4the recombination center atoms are implanted through the second surface102instead of the first surface101. The distribution of recombination center atoms obtained in this method substantially corresponds to the distributions shown inFIG. 2.

According to another embodiment (not shown) recombination center atoms are implanted through both the first surface101and the second surface102. That is, a first part of the overall implantation dose is implanted through the first surface101and a second part is implanted through the second surface102.

As stated above, activated recombination center atoms (which will briefly be referred to as recombination centers in the following) promote the recombination of charge carriers and, therefore, influence the charge carrier lifetime. In particular, recombination centers influence the minority charge carrier lifetime in a semiconductor body. A distribution of recombination centers as shown inFIG. 2may result in a lower charge carrier lifetime in regions close to the first and second surface101,102, and a higher charge carrier lifetime in regions in the middle between the first and second surfaces101,102. However, there are bipolar semiconductor devices where it is desirable to have a charge carrier lifetime in a region of one surface that is lower than the charge carrier lifetime obtained after the first diffusion process. That is, it may be desirable to have a charge carrier lifetime in the region of one surface that corresponds to the charge carrier lifetime in the middle of the semiconductor body, or is even below the charge carrier lifetime in the middle of the semiconductor body. This can be obtained by applying further method steps explained with reference toFIGS. 5A-5B.

For the purpose of explanation it is assumed that it is desired to increase the charge carrier lifetime in the region of the first surface101. However, this is only an example. It is also possible to increase the charge carrier lifetime in the region of the second surface102. In this case, the method steps applied to the first surface101and explained with reference toFIGS. 5A-5Bneed to be applied to the second surface102.

Referring toFIG. 5A, the method includes forming a doped semiconductor region11in the semiconductor body100close to or adjoining the first surface101. This doped semiconductor region11includes dopant atoms. According to one embodiment, the semiconductor body100has a basic doping of one doping type such as, for example, an n-doping before producing the doped region11, and the doping type of the doped region11corresponds to the doping type of the basic doping. According to one embodiment, the doped region11is an n-type region and the dopant atoms are phosphorus (P) atoms. A maximum doping concentration of the doped region11is, for example, between 1E19 cm−3and 1E21 cm−3. Forming the doped region11includes introducing dopant atoms into those regions where the doped region11is to be formed, and activating the introduced dopant atoms. Introducing the dopant atoms may include at least one of an implantation and diffusion process. This is explained in further detail herein below. Activating the dopant atoms includes incorporating the dopant atoms into the crystal lattice of the semiconductor body. The dopant atoms incorporated into the crystal lattice cause stress in the crystal lattice.

Referring toFIG. 5B, the temperature in a second diffusion process and the stress in the crystal lattice cause self-interstitials to form in the doped region11and to diffuse from the doped region11deeper into the semiconductor body100, that is, into those regions of the semiconductor body100that have the basic doping. The temperature in the second diffusion process is, for example, between 500° C. and the temperature of the first diffusion process. The duration of the second diffusion process is, for example, between 1 hour and 2 hours. The recombination center atoms can be introduced into the semiconductor body before or after the doped region11is produced, but, in either case, are introduced before the second diffusion process.

The self-interstitials diffusing deeper into the semiconductor body replace activated recombination center atoms (recombination centers) so that in those regions into which the self-interstitials diffuse the concentration of recombination centers is reduced by kick-out, that is, by moving recombination center atoms from substitutional sites to interstitial lattice sites where they are mobile, so that they can be gettered by the highly doped layer. This can be referred to as phosphorous diffusion gettering (PDG) when the dopant atoms are phosphorous atoms. The result of this gettering process is schematically illustrated inFIG. 6which shows the concentration of recombination centers in the semiconductor body100after the process steps explained with reference toFIGS. 5A-5B.

Referring toFIG. 6, the concentration of recombination centers is substantially 0 in the doped region11(wherein d1denotes the depth of the doped region11as seen from the first surface101) and beyond the doped region11in those regions where the self-interstitials diffuse in the second diffusion process. InFIG. 6, d2denotes the depth (as seen from the first surface101) of the region in which the concentration of recombination centers is substantially zero, where d2>d1. According to one embodiment, d2is less than 0.2d, wherein d is the thickness of the semiconductor body.

Further, adjacent the doped region11, the concentration of recombination centers may substantially correspond to the minimum concentration in the middle of the semiconductor body. That is, the concentration of recombination centers in the region adjacent the doped region11is lower than in the same regions after the second diffusion process.

There are different ways to produce the doped region11. Two embodiments are explained with reference toFIGS. 7A-7B and 8A-8Bbelow. Referring toFIGS. 7A-7B, producing the doped region11may include implanting dopant atoms through the first surface101into the semiconductor body100(seeFIG. 7A), and activating the implanted dopant atoms in an activation process so as to form the doped region11(seeFIG. 7B). The activation process may include annealing the semiconductor body100, at least in the region of the first surface101. Annealing may include heating the semiconductor body to temperatures selected from a range of between 800° C. and 1100° C., in particular between 900° C. and 950° C. for a predefined activation time. According to one embodiment, the temperature is between 900° C. and 950° C. and the duration is between 30 minutes and 120 minutes.

Referring toFIGS. 8A-8Bproducing the doped semiconductor region11may include depositing a dopant atoms including layer from a gaseous dopant source on the first surface101, and diffusing the dopant atoms from this layer via the first surface101into the semiconductor body100. InFIG. 8A, a layer220including the deposited dopant atoms is schematically illustrated. According to one embodiment, the dopant atoms are phosphorous (P) atoms, the layer220is a phosphorous glass layer and the gaseous dopant source is one of PH3and POCl3. For example, the temperature in the diffusion process is between 800° C. and 1100° C. In this method, a separate diffusion process is optional. The temperature in the deposition process is in the same range, i.e. 800° C. 1000° C., as the temperature in the diffusion process so that dopant atoms already diffuse into the semiconductor body100and are activated in the deposition process. That the method steps explained with reference toFIGS. 5A-5Bare suitable to effectively reduce the concentration of recombination centers in those regions adjoining the doped region11can be verified experimentally by measuring the forward voltage of bipolar diodes that were produced based on different semiconductor bodies, namely based on semiconductor bodies which only underwent the implantation and first diffusion process explained with reference toFIGS. 1A-1B, and based on semiconductor bodies which additionally underwent the second diffusion process explained with reference toFIG. 5B.

FIG. 9shows one embodiment a bipolar diode which is based on the semiconductor body shown inFIG. 5Band which has a distribution of recombination centers as illustrated inFIG. 6. Referring toFIG. 9, the diode includes a first emitter region adjacent a first surface101. This first emitter region is formed by the doped region11. The bipolar diode further includes a second emitter region12adjacent the second surface102, and a base region13that separates the first emitter region11and the second emitter region12. The second emitter region12has a doping type complementary to the doping type of the first emitter region11and a higher doping concentration than the base region13. The doping concentration of the base region13may correspond the basic doping of the semiconductor body100. Optionally, the bipolar diode further includes a field-stop region14of the same doping type as the first emitter region11and more highly doped in the base region13. According to one embodiment, the field-stop region14adjoins the first emitter region11. Referring to another embodiment (not shown) the field-stop region14is distant to the first emitter region11.

Referring toFIG. 9, the bipolar diode further includes a first load terminal21connected to the first emitter region11, and a second load terminal22connected to the second emitter region12. For the purpose of explanation it is assumed the first emitter region11is n-doped and the second emitter region12is p-doped. In this case, first load terminal21is a cathode terminal and the second load terminal22is an anode terminal of the bipolar diode. Further, it is assumed that the base region13has the same doping type as the first emitter region11so that a pn-junction15is formed between the second emitter region12and the base region13.

Depending on a voltage applied between the first and second load terminals21,22the bipolar diode is either conducting (in a conducting mode) or blocking (in a blocking mode). The bipolar diode is conducting when the voltage applied between the first and second load terminals21,22has a polarity that forward biases the pn-junction15, and the bipolar diode is blocking when the voltage applied between the first and second load terminals21,22has a polarity that reverse biases the pn-junction15.

When the bipolar diode is conducting the second emitter region11injects charge carriers of a first type into the base region13and the second emitter region12injects charge carriers of a second type complementary to the first type into the base region13. The charge carriers of the first type are electrons when the first emitter region11is n-doped, and the second charge carriers are holes when the second emitter region12is p-doped. The charge carriers injected by the first and second emitter regions11,12into the base region13form a charge carrier plasma in the base region13.

When the bipolar diode switches from the conducting mode to the blocking mode, a space charge region (depletion region) expands beginning at the pn-junction15in the base region13. This depletion region expanding in the base region13causes the charge carrier plasma to be removed from the base region13, wherein a current flows through the bipolar diode until the charge carrier plasma is completely removed from the base region13.

The amount of charge carriers in the charge carrier plasma when the bipolar diode is conducting can be adjusted by adjusting the minority charge carrier lifetime in the base region13. A stated above, the minority charge carrier lifetime can be adjusted by providing recombination centers in the base region13. Basically, the higher the concentration of recombination centers, the lower the minority charge carrier lifetime, and the smaller the amount of charge carriers in the charge carrier plasma. Thus, the smaller the amount of charge carriers in the charge carrier plasma, the lower are reverse recovery losses, which are the losses that occur when the bipolar diode switches from the conducting mode to the blocking mode. The higher the amount of charge carriers in the charge carrier plasma, the longer it takes for the charge carrier plasma to be removed from the base region13and the longer a current flows through the bipolar diode when the bipolar diode switches from the conducting mode to the blocking mode.

At the end of the reverse recovery process, that is, at the end of the process in which the charge carrier plasma is removed from the base region13the current through the bipolar diode turns to zero. Since abrupt changes of the current through the bipolar diode may cause voltage peaks in inductances in a circuit (not shown) connected to the bipolar diode it may be desirable for the current at the end of the reverse recovery process to “softly” turn to zero. This can be obtained by suitably adjusting the minority charge carrier lifetime in those regions of the base region12that are depleted towards the end of the reverse recovery process. In the bipolar diode shown inFIG. 9, these are the regions distant to the pn-junction15and close to the first emitter region11and the field-stop region14, respectively. In particular, the charge carrier lifetime is adjusted such that the charge carrier lifetime in those regions of the base region13which are depleted towards the end of the reverse recovery process corresponds to the minority charge carrier lifetime or is even higher than the minority charge carrier lifetime in those regions closer to the pn-junction15and depleted at the beginning of the reverse recovery process.

Referring toFIG. 6, the method explained with reference toFIGS. 1A-1B and 5A-5Bresults in a semiconductor body that meets these requirements concerning the charge carrier lifetime. This has been verified by producing six samples of bipolar diodes that are based on differently processed semiconductor bodies. The following features of the bipolar diode were identical in each of the samples:

1E13 (specificBasic n-doping of the semiconductor bodyresistance:(doping concentration of the base region 13)450 Ω cm)Thickness d of the semiconductor body 100520μmp-dopant dose of the second emitter region 123E13cm−2Depth d2 of the second emitter region 12 (distance6μmbetween the second surface 102 and the pn-junction 15)n-dopant dose of the first emitter region 115E15cm−2Depth d1 of the first emitter region 110.8μmMaximum doping concentration of the first emitter2E20cm−3region 11Maximum doping concentration of the field-stop1E15cm−3region 14Depth of the field-stop region 14 from the first30μmsurface 101

The individual samples were different in view of producing the recombination centers (adjusting the minority charge carrier lifetime). In four (samples 1-4) of the six samples, platinum atoms were implanted and diffused as explained with reference toFIGS. 1A-1B, while in two (samples 5-6) of the six samples platinum atoms were diffused from a platinum silicide in a conventional way. Further, in three of the six samples (samples 2, 4, 6) a second diffusion process was performed as explained with reference toFIGS. 5A-5B, while in the other three samples (samples 1, 3, 5) the second diffusion process was omitted. In case of sample 6, the platinum silicide was removed before the second diffusion process.

The details of producing the recombination centers in the individual semiconductor bodies are summarized below.

Sample 1Way of introducing platinum atomsimplantationImplantation dose1E12cm−2Temperature of the first diffusion825°C.processTemperature of the second diffusion—process(no second diffusion process)

Sample 2Way of introducing platinum atomsimplantationImplantation dose1E12cm−2Temperature of the first825°C.diffusion processTemperature of the second600°C.diffusion process

Sample 3Way of introducing platinum atomsimplantationImplantation dose2E12cm−2Temperature of the first diffusion process825°C.Temperature of the second diffusion—process(no second diffusion process)

Sample 4Way of introducing platinum atomsimplantationImplantation dose1E12cm−2Temperature of the first diffusion process825°C.Temperature of the second diffusion600°C.process

Sample 5diffusion from a platinumWay of introducingsilicide formed on one of theplatinum atomssurfaces of the semiconductor bodyImplantation dose—Temperature of the first795° C.diffusion processTemperature of the second—diffusion process(no second diffusion process)

Sample 6:diffusion from a platinumWay of introducingsilicide formed on one of theplatinum atomssurfaces of the semiconductor bodyImplantation dose—Temperature of the first795° C.diffusion processTemperature of the second600° C.diffusion process

Recombination centers formed in the semiconductor body, in particular in the base region13, not only reduce the minority charge carrier lifetime, but also increase the forward voltage VFof the bipolar diode. The “forward voltage” is the voltage between the first and second load terminals21,22at a rated current flowing through the bipolar diode when the bipolar diode is in the conducting mode.

In the diodes according to samples 1-6 explained above the forward voltage was measured at a rated current of 100 A. The forward voltages obtained in these measurements are illustrated inFIG. 10. Referring to the results obtained for samples 1 and 3, which are only different in the implantation dose of the platinum atoms, a higher implantation dose and, therefore, a higher concentration of recombination centers results in a higher forward voltage (the forward voltage of sample 3 is higher than the forward voltage of sample 1). The same can be seen from samples 2 and 4 which are only different in the implantation dose (the forward voltage of sample 4 is higher than the forward voltage of sample 2). Further, the forward voltage of sample 2 is lower than the forward voltage of sample 1, wherein samples 1 and 2 are only different in that a second diffusion process as explained with reference toFIGS. 5A-5Bwas performed in sample 2. This strongly indicates that the second diffusion process results in a reduction of recombination centers in the base region13in those regions in which self-interstitials diffuse from the doped region11. The same result can be seen by comparing samples 3 and 4 which are only different in that a second diffusion process was performed in sample 4; the forward voltage VFobtained for sample 4 is lower than the forward voltage VFobtained for sample 3.

In the conventional process, that is the process of samples 5-6, no significant differences between the forward voltages was monitored. This strongly indicates that in the conventional process the concentration of recombination centers close to the surfaces of the semiconductor body is so high that no significant reduction of the recombination centers can be obtained by the second diffusion process.

Thus, if the surface concentration of the recombination centers is below the solubility limit, then the concentration of recombination centers can be effectively reduced by diffusing self-interstitials from the doped region11deeper into the semiconductor body. Recombination center concentrations below the solubility limit can be obtained by implanting recombination center atoms into the semiconductor body instead of diffusing the recombination center atoms from an alloy. In particular, implanting the recombination center atoms may include implanting the recombination center atoms with an implantation dose such that all (100%) of the implanted recombination center atoms are activated at a given temperature. This temperature is, for example, between 700° C. and 1000° C., in particular between 750° and 950° C.

Further, by suitably adjusting the process parameter in the process of implanting the recombination center atoms and diffusing the implanted recombination center atoms the temperature coefficient of the forward voltage can be adjusted. This is explained with reference toFIGS. 11-13below.

FIG. 11shows the forward voltage VFand the corresponding forward current IFof an example bipolar diode at a first temperature T1such as, for example, 25° C. and a second temperature T2such as, for example, 125° C. As can be seen fromFIG. 11, the forward voltage VFincreases as the forward current IFincreases. The forward current IFis, for example, a current driven through the bipolar diode by a load (not shown in the figures) connected in series with the bipolar diode. The relationship between the forward voltage VFand the forward current IFis dependent on the temperature. That is, at different temperatures the forward voltage VFincreases differently as the forward current increases. Generally, it is desirable for the forward voltage VFto have a positive temperature coefficient. A “positive temperature coefficient” means that at a given forward current IFwhich is equal to or higher than the rated current the forward voltage VFis the higher, the higher the temperature is. A “positive temperature coefficient” also means that at a given forward voltage VFthe forward current IFis the lower, the higher the temperature is.

For example, a positive temperature coefficient of a bipolar diode is beneficial in a circuit application in which several bipolar diodes are connected in parallel. If each of the bipolar diode has a positive temperature coefficient and if the temperature of one of these bipolar diodes becomes higher than the temperatures of the other bipolar diodes, then the current through the bipolar diode with the higher temperature decreases. This has the effect that the power dissipated in the bipolar diode with the higher temperature decreases, so as to counteract a further increase of the temperature of this bipolar diode. In case of a negative temperature coefficient of the bipolar diodes, a higher temperature of one bipolar diode would result in an increasing current through this bipolar diode which, in turn, would result in an increase of power dissipated in the bipolar diode with the higher temperature, which would further increase the temperature of this bipolar diode and may finally cause the bipolar diode to be damaged or destroyed.

In the example shown inFIG. 11, the forward voltage VFhas a positive temperature coefficient at operation points above a temperature stable operation point. Each operation point is defined by the forward voltage VFand the corresponding forward current IFat one temperature. The temperature stable operation point is defined by forward voltage VF0and a corresponding forward current IF0, wherein the forward voltage VFis independent of the temperature in this operation point. At operation points below this temperature stable operation point VF0, IF0, the temperature coefficient is slightly negative.

FIG. 12illustrates an example in which the forward voltage VFhas a negative temperature coefficient. In this example, for each forward voltage VFthe forward current IFincreases as the temperature increases.

Experiments have shown that the temperature behavior of the forward voltage VFis dependent on the concentration of recombination centers in the base region13close to the pn junction15. This is explained below.

In the conventional process explained before, the concentration of recombination centers can only be varied by varying the temperature at which the recombination centers diffuse from the alloy into the semiconductor body. In this conventional process, the surface concentration always corresponds to the solubility limit. In a power diode, such as a power diode explained with reference to theFIG. 9, the pn junction15is usually close to the surface. That is, a distance of the pn junction15to the nearest surface is usually less than 2% of the thickness of the semiconductor body100. For example, in a (silicon) power diode with a voltage blocking capability of 4.5 keV the thickness d of the semiconductor body is about 500 micrometers while the distance of the pn junction to the second surface102is only about 6 micrometers, which is 1.2% of the thickness of the semiconductor body. Thus, the concentration of recombination centers in the region of the pn junction can be considered to substantially correspond to the surface concentration.

As stated above, the overall amount of recombination centers in the base region13affects the forward voltage, wherein the forward voltage VFincreases as the overall amount of recombination centers increases. In the conventional process, the amount of charge carriers and, therefore, the forward voltage at a rated current can only be adjusted by the temperature. However, experiments have shown that producing the recombination centers in the conventional process results in a slightly negative temperature coefficient of the forward voltage.

While in the conventional method there is only one parameter that can be varied in order to adjust the overall amount of recombination centers in the base region, the method explained with reference toFIGS. 1A-1Bincludes two parameters that can be varied, namely the implantation dose of the recombination center atoms and the temperature in the first diffusion process. This is explained with reference toFIG. 13below.

FIG. 13shows a curve that illustrates the implantation dose of recombination center atoms and the corresponding temperature in the first diffusion process in order to obtain a predefined amount of recombination centers in the base region13of a bipolar diode. This predefined amount of recombination centers corresponds to a predefined forward voltage at a rated current. The curve shown inFIG. 13was based on a bipolar diode with the following parameters:

1E13 (specificBasic n-doping of the semiconductor bodyresistance:(doping concentration of the base region 13)450 Ω cm)Thickness d of the semiconductor body 100520μmp-dopant dose of the second emitter region 123E13cm−2Depth d2 of the second emitter region 12 (distance6μmbetween the second surface 102 and the pn-junction 15n-dopant dose of the first emitter region 115E15cm−2Depth d1 of the first emitter region 110.8μmDopant dose of the field-stop region 148E11cm−2Depth of the field-stop region 14 from the first60μmsurface 101
At a rated current of, for example, 100 A this bipolar diode has a forward voltage VFof about 1.8V when no recombination centers are produced. Producing recombination centers increases the forward voltage VF. In the experiment underlying the curve shown inFIG. 13recombination centers were produced in the base region to such an extent that the forward voltage VFat the rated current was about 2.8V. In the example, the semiconductor body100includes silicon (Si) and the recombination center atoms are platinum (Pt) atoms.

In the following, the implanted dose of recombination centers and the corresponding temperature of the first diffusion process that result in a particular amount of recombination centers (a particular forward voltage VF) will be referred to as parameter pair. InFIG. 13, parameter pairs that result in the same forward voltage (about 2.8 V in this example) are represented by the curve shown in the figure. Referring toFIG. 13, there is a plurality of parameter pairs that result in the same amount of recombination centers in the base region. The diffusion temperature increases as the implantation dose decreases. That is, at lower implantation doses higher temperatures are required to activate the desired amount of recombination center atoms. In other words, at higher implantation doses the corresponding temperature is smaller so that at higher implantation doses a smaller portion of the implanted recombination center atoms is activated in the base region than at lower implantation doses. In the experiment, the implantation dose was varied between 1E12 cm−2and 1E13 cm−2resulting in a variation of the temperature between about 750° C. and about 815° C. The dashed line inFIG. 13represents the temperature (about 795° C.) that is required in the conventional process to diffuse in and activate recombination center atoms to such an extent that the forward voltage VFis about 2.8V.

Measurements have shown that the forward voltage VFof the diode produced in accordance with the standard process has a negative temperature coefficient (TC). The same applies to diodes produced with first diffusion temperatures higher than the temperature of the conventional process and relatively low implantation dose. However, at lower temperatures such as, for example, temperatures below 850° C., in particular below 790° C. and higher implantation doses such as, for example, higher than 3E12 cm−2a positive temperature coefficient (TC) of the forward voltage was detected. One possible reason for this is explained below.

The implantation of recombination center atoms into the semiconductor body100causes implantation damages, so-called point defects. Those defects mainly occur in the region of that surface into which the recombination center atoms are implanted and the concentration of those defects increases as the implantation dose increases. Referring to the explanation above, the recombination center atoms can be implanted into the semiconductor body100via the first surface101or the second surface102. However, in the first diffusion process, these defects rapidly diffuse in the semiconductor body100so that point defects can be found in the region of the implantation surface (which is the surface into which the ions are implanted) as well as in the region of the opposite surface. Thus, referring to the embodiments explained with reference toFIGS. 4 and 9, point defects in the second emitter region12can also be generated by implanting the recombination center atoms into the first surface101opposite the second surface102.

These point defects enable platinum to be substitutionally incorporated into the crystal lattice of the semiconductor body100. In particular, these point defects may result in a concentration of incorporated (activated) platinum atoms in the region of the first and second surface101,102which is above the solubility limit at the temperature of the first diffusion process, such as above the solubility limit at a temperature of 790° C. or higher. “In the region of the first and second surfaces101,102” means in a region close to these first and second surfaces102. That is, for example, in a region having a width of between 500 nanometers and 1 micrometer beginning at the respective surface101,102. For example, in a bipolar diode as shown inFIG. 9, the high platinum concentration in the region of the second surface, that is, in the region of the second emitter12reduces the efficiency of the second emitter12, such that at a given (rated) current a voltage drop across the second emitter12increases as the operation temperature increase. This results in a positive temperature coefficient, while the increased voltage drop across the second emitter12does not significantly increase the forward voltage which is mainly defined by the base region13and the concentration of recombination center atoms therein.

The above explanation with regard toFIG. 13may be summarized as follows. Referring toFIG. 13, the forward voltage VFsubstantially is the same, that is, the doping concentration of the base region (13inFIG. 9) substantially is the same when the implantation dose increases but the temperature of the first diffusion process decreases. However, the higher implantation dose in connection with the higher concentration of implantation defects (which supposedly agglomerate in the region of the surfaces101,102) may result in concentrations of recombination centers in the region of the surfaces101,102which are above the solubility limit at the temperature of the diffusion process. These high concentrations, in turn, reduce the efficiency of the emitter (the second emitter12inFIG. 9) which has a doping type complementary to the doping type of the base region13and, therefore, result in a positive temperature coefficient.

It has been verified by experiments, that implanting recombination center atoms followed by a first temperature (diffusion) process may result in a concentration of recombination center atoms which is higher than the solubility limit at the temperature of the first temperature process. The result of one of these experiments is shown inFIG. 14. In this experiment, platinum atoms were diffused into a semiconductor body in accordance with a conventional diffusion process at a diffusion temperature of 790° C. The surface concentration obtained through this is illustrated in dashed lines inFIG. 14(see the graph labeled “Standard 790° C.” inFIG. 14). Further, in this experiment, platinum atoms were introduced in a comparable semiconductor body by implanting the platinum atoms and diffusing the implanted atoms at the same temperature as the conventional process, that is, at 790° C. The implantation dose was selected such that in an inner region of the semiconductor body, that is, a region corresponding to the base region in the embodiments explained herein before the concentration of recombination center atoms was substantially the same in both processes. Referring toFIG. 14a significantly higher platinum concentration can be obtained by implanting and diffusing instead of diffusing, only. While the platinum concentration at the surface is about 2E14 cm−3(which corresponds to the solubility limit at 790° C.) in the conventional process it is about 3E15 cm−3in the implantation and diffusion process.

The implantation process explained with reference toFIG. 1Acan be performed such that the recombination center atoms are implanted into the semiconductor body100all over the first surface101. In this case, after the first diffusion process, there may be a vertical variation of the recombination center concentration as shown inFIG. 2. That is, the concentration of recombination centers may vary in the vertical direction of the semiconductor body100which is a direction perpendicular to the first surface. However, in a lateral direction, which is a direction parallel to the first surface101there is substantially no variation of the recombination center concentration.

According to one embodiment shown inFIG. 15the recombination center atoms are implanted into the first surface101of the semiconductor body100using an implantation mask300. The implantation mask covers sections of the first surface101and prevents recombination center atoms from being implanted in those regions of the first surface101covered by the implantation mask300. In this process, a lateral variation of the recombination center concentration can be obtained. In particular, there can be regions of the semiconductor body100below the implantation mask300where, after the first diffusion process, the concentration of recombination centers is substantially zero. In the first diffusion process, the implanted recombination center atoms not only diffuse in the vertical direction of the semiconductor body100but also in the lateral direction. However, the maximum diffusion length in the lateral direction substantially can be adjusted by the thermal budget involved in the diffusion process, that is, by the temperature and the duration in the first diffusion process. According to one embodiment, the thermal budget is selected such that the diffusion width in the lateral direction substantially corresponds to the thickness d of the semiconductor body. Thus, dependent on the size of the implantation mask300there can be regions in the semiconductor body100where no recombination centers are produced. According to one embodiment, the temperature process is an RTA (Rapid Thermal Annealing) process.

InFIG. 15, the dotted line schematically illustrates the situation after the first diffusion process. The dotted line represents a border between those regions121of the semiconductor body100in which recombination center atoms diffuse from the end of range in the first diffusion process and those regions122in which recombination center atoms neither are implanted nor diffuse.

Such lateral variation of the recombination center atoms may be used in a variety of different semiconductor device. For example, the method can be used to adjust recombination centers substantially only in an edge region of a semiconductor device. This is explained with reference toFIG. 16.

FIG. 16illustrates a vertical cross sectional view of a bipolar diode integrated in a semiconductor body100. The semiconductor body includes an inner region131in which the first and second emitter regions11,12and the base region13are arranged, and an edge region132that surrounds the inner region131. The edge region132may adjoin an edge surface of the semiconductor body (as illustrated inFIG. 16). However according to another embodiment, the edge region132separates the inner region131with the active device regions (emitter regions11,129) from other devices integrated in the same semiconductor body. According to one embodiment, the method explained with reference toFIG. 14is used to generate recombination centers in the edge region132. In this embodiment, recombination center atoms are implanted via the first surface101or the second surface102into the semiconductor body100with an implantation mask300covering the inner region131and leaving uncovered the edge region. Such implantation mask is schematically illustrated on the second surface102inFIG. 16. In the implantation process a scattering layer (not shown inFIG. 15) can be used. After the implantation process, the implanted recombination center atoms diffuse in the semiconductor body100substantially in the edge region132so as to produce recombination centers in the edge region. The process parameters such as implantation dose, implantation energy, and diffusion temperature may correspond to the parameters explained before.

It should be noted that the methods explained with reference toFIGS. 1A-1B and 15can be combined. That is, the method explained with reference toFIG. 1A-1Bmay be used to generate recombination centers all over the semiconductor body100while additionally the method explained with reference toFIG. 14may be used to additionally produce recombination centers in selected regions of the semiconductor body100.

The method explained with reference toFIG. 15is not restricted to be used for producing recombination centers in an edge region of a semiconductor device, but may be used in each case where it is desired to selectively produce recombination centers in certain regions of a semiconductor body while omitting other regions of the semiconductor body. For example, an IGBT and a diode are integrated in one semiconductor body and recombination centers are only produced in those regions that include the diode.

Referring to the above, forming the doped region11may include an implantation and/or a diffusion process at temperatures higher than 950° C. and durations of between 30 minutes and 100 minutes. If those temperatures, which are above the temperature range of the first diffusion process, would be applied to the semiconductor body100for a relatively long time, such as more than several minutes, after the recombination center atoms had been implanted and diffused in the first diffusion process substantially all recombination center atoms would be removed by the gettering effects explained above. That is, at those high temperatures self-interstitials would diffuse deep into the semiconductor body100and replace the recombination center atoms at substitutional sites of the crystal lattice. These self-interstitials result from introducing the dopant atoms that finally form the doped region11. In order to prevent this, the doped region11may be formed before implanting the recombination center atoms and before the first diffusion process. The optional second diffusion process, which may include temperatures of between 500° C. and the maximum temperature in the first diffusion process and which may be used to reduce the recombination center concentration close to the doped region11, may then be performed after forming the doped region and after the first diffusion process.

According to another embodiment, the doped region11is formed after implanting the recombination center atoms and the first diffusion process, but before the optional second diffusion process. The process of forming the doped region11before implanting the recombination center atoms and the first diffusion process may include implanting the dopant atoms into the first surface101, as explained with reference toFIG. 7A, and one of a laser annealing and an RTA (Rapid Thermal Annealing) process to activate the implanted dopant atoms.

The laser annealing process includes heating by laser pulses the semiconductor body in the region of the first surface101such that the semiconductor material in this region melts and afterwards recrystallizes, whereas during recrystallization the introduced dopant atoms are incorporated into the crystal lattice and thereby activated. The doping concentration of the doped region11obtained by this method is substantially homogenous, whereas the doping level is dependent on the dopant dose introduced in the implantation process and the depth of the doped region11. The “depth” of the doped region is the dimension the doped region11extends into the semiconductor body100from the first surface101. For example, a dopant dose is selected from a range of between 1E15 cm−2and 1E16 cm−2. The depth of the doped region11is, for example, in a range of between 400 nanometers and 1000 nanometers (=1 micrometer). If, for example, the dopant dose is 4E15 cm−2and the depth of the emitter is 400 nanometers, a doping concentration of the doped region11of 1 E22 cm−3(=4E15 cm−2/4E-7 cm) may result if 100% of the introduced dopant atoms are activated. However, dependent on the specific type of the laser annealing process only between 30% and 80% of the introduced dopant atoms may become electrically active.

The depth of the doped region11can be adjusted by suitably selecting a wavelength of the laser light and the laser energy in the laser annealing process. In order for the laser energy to be absorbed in a region close to the first surface101laser light with a relatively short wavelength may be used. According to one embodiment, the semiconductor body100comprises silicon, and the wavelength of the laser light is selected from a range of between 300 nanometers and 350 nanometers. The laser energy is, for example, selected from a range of between 3 J/cm2and 4 J/cm2. The relatively short wavelength of the laser light mentioned before causes the laser energy to be absorbed substantially within a region that extents between 20 nanometers and 30 nanometers from the first surface101into the semiconductor body. The region where the semiconductor body100melts and recrystallizes extends deeper than those 20-30 nanometers and may extend between 300 nanometers and 500 nanometers. The laser light is applied to the first surface101in laser pulses, whereas a duration of those laser pulses may be selected such that in the semiconductor body melts for less than several 100 nanoseconds (ns) and then recrystallizes. According to one embodiment, the semiconductor body, in the region of the first surface101, is melted for the less than 300 nanoseconds and then recrystallizes. For example, the duration of one laser pulse is about 150 ns. The laser annealing process may include applying a plurality of laser pulses to the first surface101, wherein each of these laser pulses is directed to another section of the first surface101until a desired area of the first surface101has been melted and recrystallized. The individual sections may slightly overlap. Operating the laser in this way is referred to as stitched mode. This desired area may include the complete surface101or, alternatively, may include only a section of the first surface101.

In the laser annealing process, surface region of the semiconductor body100are heated for such a short time period that substantially no diffusion or gettering of the recombination centers introduced before may occur in this time period. “Surface regions” of the semiconductor body100are regions adjacent the first surface101of the semiconductor body100.

According to one embodiment, the laser annealing process includes forming an anti-reflective coating (ARC) layer300on the first surface101and directing the laser light in the laser annealing process to this ARC layer300. This is schematically illustrated inFIG. 17.

FIG. 17shows a vertical cross sectional view of the semiconductor body100after introducing the dopant atoms and before the laser annealing process. Reference character11′ denotes those regions of the semiconductor body100adjacent the first surface into which dopant atoms where introduced and which is melted by the laser annealing process so as to form the doped region11. The ARC layer300helps to increase the efficiency of the laser annealing process by avoiding or at least reducing reflections of the laser light at the first surface101. This reduces losses by reflection and increases the amount of laser energy dissipated in the surface region.

According to one embodiment, the ARC layer300includes one of a silicon oxide (SiO2) layer and a silicon nitride (Si3N4) layer. The use of an ARC layer300may cause the melted zone (the zone where the semiconductor body is melted) to extend deeper from the first surface101than in a method where no ARC layer300is used. This is explained with reference toFIG. 18below.

FIG. 18shows the doping concentration in a doped region11obtained by three different methods. InFIG. 18, curve401represents a doping profile obtained by a laser annealing process without ARC layer300, curve402represents the doping profile obtained by a laser annealing process using a silicon oxide ARC layer300and, for comparison, curve403represents the doping profile obtained by an implantation and diffusion process as explained with reference toFIGS. 7A-7B. The curves401-403shown inFIG. 18are based on three different experiments, wherein the implantation dose was the same in each of these three experiments. Referring of the above, the implantation dose may be selected from a range of between 1E15 cm−2and 1E16 cm−2. In the experiments underlying the doping profiles shown inFIG. 18the dopant dose was about 5E15 cm−2. By integrating the doping concentration according to the different doping profiles401-403it can be shown that the overall number of activated dopant atoms is substantially the same in each of the three cases. In this specific embodiment, the integral of the doping concentration is about 2E15 cm−2in each case, so that in this case about 40% of the implanted dopant atoms have been activated.

As can be seen fromFIG. 18, the doped region11obtained by the laser annealing process with ARC layer (curve402) extends deeper into the semiconductor body100than the doped region11obtained by the laser annealing process without ARC layer (curve401). The laser annealing processes (curves401,402) result in sharper doping profiles than the implantation and diffusion process (curve403). Furthermore, the doped region11formed by a laser annealing process extends less deep into the semiconductor body100than the doped region11obtained by the implantation and diffusion process. The surface concentration, which is the doping concentration of the doped region11at the first surface101is substantially the same in each of the three methods.

An RTA process, as an alternative to a laser annealing process, may include heating the first surface101to a temperature selected from a range of between 800° C. and 1050° C., in particular between 900° C. and 950° C. for an activation time of between 5 seconds and 5 minutes. In particular, the activation time is selected from between 10 seconds and 3 minutes. Heating the first surface101may include heating the first surface101by one of a lamp and a laser. By virtue of the relatively short activation period, a diffusion or gettering of the recombination centers can be widely prevented.

According to one embodiment, the method, after forming the recombination centers in the semiconductor body100, includes reducing the thickness of the semiconductor body100by removing a layer of semiconductor material at at least one of the first and the second surface101,102. Removing the semiconductor material may include at least one of an etching process, a mechanical polishing process, or a chemical-mechanical polishing process (CMP). Referring toFIG. 2, forming the recombination centers in accordance with one of the methods explained before results in a maximum of the recombination center concentration close to the first and the second surface101,102. That is, the recombination center concentration increases towards the first and the second surface101,102. By removing semiconductor material at the first and/or surface101,102those sections of the semiconductor body100are removed where the maximum of the recombination center concentration is. Through this, a more homogenous distribution of recombination centers in the semiconductor body100can be obtained. According to one embodiment the thickness of the removed semiconductor material is selected from a range of between 200 nanometers and 2 micrometers, in particular between 500 nanometers and 1 micrometer. Such material layer may be removed at only one of the first and second surfaces101,102or at both surfaces101,102.

Forming the doped region11after forming the recombination centers in the semiconductor body100can be beneficial in several ways. First, it allows to remove semiconductor material at at least one of the first and second surfaces101,102to obtain a more homogenous distribution of the recombination centers. Second, catalytic effects of the doped region11on the generation of recombination centers can be prevented. Experiments have given cause to the assumption that the presence of the doped region11at the time of introducing and activating the recombination center atoms causes a segregation and pile up of recombination center atoms at the first surface101. This may negatively influence the forward voltage of the device. Third, the recombination center atoms may form pair complexes with the dopant atoms from the doped region11. For example, if the recombination center atoms are platinum atoms and the dopant atoms in the doped region11are phosphorous atoms so-called phosphorous-platinum pairs may be formed. The presence of those pair complexes may affect, in particular decrease, the forward voltage. As segregation and the formation of pair complexes is difficult, if not impossible to predict segregation and/or pair complexes may result in unpredictable variations of the forward voltage. That is, a group of devices formed by the same process may exhibit a large standard deviation of the forward voltage.

Referring toFIG. 9, the semiconductor device may include a field-stop region14. The field-stop region14may adjoin the doped region11(as shown) or may be spaced apart from the doped region11. Forming the field-stop region may include implanting protons (hydrogen ions) via the first surface101and an annealing process at temperatures of between 220° C. and 500° C. and a duration of between 30 minutes and 10 hours. In the annealing process n-doping complexes form in those regions into which the protons had been implanted. Those n-doping complexes are referred to as hydrogen induced donors. As the temperature in this annealing process is lower than the temperatures in the first diffusion process and the second diffusion process, the field-stop region may be formed after these diffusion processes. According to one embodiment, the field-stop region14is formed to have a maximum doping concentration of 1E15 cm−3.

Although the semiconductor device shown inFIG. 9is a diode the method explained herein above is not restricted to be used in processes for manufacturing diodes. Moreover, the device structure with the base region13, the doped region11, the optional field-stop region14and the recombination centers at least in the base region may be used in any type of bipolar devices. Examples of those further bipolar devices include, but are not restricted to, IGBTs (Insulated Gate Bipolar Transistors), in particular RC (Reverse Conducting) IGBTs, and GTO (Gate Turn Off) thyristors.

By the method explained herein before a relatively homogenous concentration of the recombination center atoms in the vertical direction x of the semiconductor body100can be obtained. Relatively homogenous means that a ratio between a maximum concentration NEin a region to one of the first and second surfaces101,102and a minimum concentration NMin the middle of the semiconductor body100is less than 200, less than 100, less than 50, or even less than 10, that is NE/NM<200, NE/NM<100, NE/NM<50, or even NE/NM<10. Referring toFIGS. 2 and 6, the semiconductor body100may include two maxima (see,FIG. 2), one in the region of each surface101,102, or only one maximum (see,FIG. 6), namely in the region of one surface while the other maximum was removed by PDG. In case there are two maxima, NEdenotes the concentration of the greater one of the two maxima. The minimum “in the middle of the semiconductor body100” is the minimum spaced apart from those region affected by the PDG process. According to one embodiment, the minimum in the middle is distant more than 0.2d from each of the surfaces101,102. According to one embodiment, the minimum is in a region of between 0.3d and 0.7d from one of the first and second surfaces101,102, wherein d denotes the thickness of the semiconductor body100. If there is an optional thinning process, d denotes the thickness after the thinning process.