Semiconductor device producing method

A first laser pulse emitted from a semiconductor laser oscillator and having a first pulse width is entered onto a second surface of a semiconductor substrate in which a semiconductor device is formed on a first surface and dopants are added to a surface layer portion on the second surface side. A second laser pulse having a second pulse width less than or equal to 1/10 of the first pulse width is entered on an incident area of the first laser pulse in an overlapping manner. The relative positional relationship on a time axis between falling time of the first laser pulse and rising time of the first laser pulse is set such that the temperature of the first surface, which rises due to the incidence of the first laser pulse and the second laser pulse, does not exceed an allowable upper limit value which is predetermined.

RELATED APPLICATIONS

Priority is claimed to Japanese Patent Application No. 2012-176435, filed Aug. 8, 2012, and International Patent Application No. PCT/JP2013/067255, the entire content of each of which is incorporated herein by reference.

BACKGROUND

Technical Field

The present invention relates to a semiconductor device producing method in which dopants implanted into a semiconductor substrate are activated by the irradiation with a laser beam.

Related Art

Japanese Unexamined Patent Application Publication No. 2010-171057 (PTL1) discloses a semiconductor device producing method in which an electrode layer and a field stop layer are formed by introducing dopants into a semiconductor substrate by ion implantation and then activating the dopants by the irradiation with a laser beam. In the technique described in PTL 1, after a surface structure of a semiconductor device is fabricated, the substrate is thinned. Thereafter, ion implantation is performed on a rear surface and the rear surface is irradiated with pulsed laser beams by using two laser oscillators. A temporal difference between laser pulses which are emitted from the two laser oscillators is set to be less than or equal to 600 ns.

Specifically, for example, phosphorus ions are ion-implanted into a field stop layer formation planned portion from the rear surface side of an n−type silicon substrate. At this time, a dose amount is set to be less than or equal to 1×1014cm−2such that the field stop layer has peak concentration of less than or equal to 5×1018cm−3. Subsequently, for example, boron ions and phosphorus ions are respectively implanted into a p+type collector layer formation planned portion and an n+type cathode layer formation planned portion with a dose amount set to be less than or equal to 5×1016cm−2such that the a p+type collector layer and an n+type cathode layer have peak concentration of less than or equal to 1×1021cm−3.

It is known that the n+type cathode layer or the like in which ions are implanted at such high concentration is amorphized due to breaking of the crystallinity of the silicon substrate. In the semiconductor device producing method described in PTL 1, defect recovery by solid-phase diffusion and the activation of the dopants implanted into a deep portion exceeding a depth of 1 μm from a laser irradiated surface (the rear surface of the substrate) are performed using the laser irradiation. However, sufficient temperature rise and heating time cannot be secured, and thus the activation of the dopants may be insufficient. If the pulse energy density of the laser beam which is input is increased in order to sufficiently perform the activation of the deep portion, a melting depth becomes deep. If melting is made to the deep portion, dopant concentration distribution in a depth direction changes, and thus characteristics as designed cannot be obtained in some cases. In addition, a problem occurs such as the roughness of the substrate surface becoming worse.

A technique of performing laser annealing by emitting a pulsed laser beam at a pulse energy density in the range in which a semiconductor substrate is not melted is proposed (refer to Japanese Unexamined Patent Application Publication No. 2009-32858, for example). In this method, dopants are ion-implanted at high concentration, and thus it is difficult to sufficiently recover the crystallinity of an amorphized portion and sufficiently activate the implanted dopants.

SUMMARY

According to an aspect of the present invention, there is provided a semiconductor device producing method including:

entering a first laser pulse emitted from a semiconductor laser oscillator and having a first pulse width onto a second surface of a semiconductor substrate in which a semiconductor device is formed on a first surface and dopants are added to a surface layer portion on the second surface side; and

entering a second laser pulse having a second pulse width less than or equal to 1/10 of the first pulse width on an incident area of the first laser pulse in an overlapping manner,

wherein a relative positional relationship on a time axis between falling time of the first laser pulse and rising time of the first laser pulse is set such that a temperature of the first surface, which rises due to incidence of the first laser pulse and the second laser pulse, does not exceed an allowable upper limit value which is predetermined.

DETAILED DESCRIPTION

In the above mentioned related art, at the point in time when the dopants implanted into the rear surface of the semiconductor substrate are activated, the surface structure of the semiconductor device is already formed on the front side. If the laser irradiation for the activation of the dopants and the recovery of the crystallinity is performed on the rear surface, the temperature of the front side also rises. A significant rise in the temperature of the front side causes damage to the surface structure already formed.

It is desirable to provide a semiconductor device producing method in which when performing activation of dopants by irradiating a surface on one side with a laser, it is possible to suppress a rise in the temperature of a surface on the opposite side.

InFIG. 1, a schematic diagram of a laser annealing apparatus which is used in a semiconductor device producing method according to an embodiment is shown. A semiconductor laser oscillator (a first laser oscillator)21emits, for example, a quasi-continuous wave (QCW) laser beam having a wavelength of 808 nm. A semiconductor laser oscillator which emits a pulsed laser beam having a wavelength of less than or equal to 950 nm may be used. A solid-state laser oscillator (a second laser oscillator)31emits a pulsed laser beam having a green wavelength region. For example, an Nd:YAG laser, an Nd:YLF laser, an Nd:YVO4laser, or the like which emits a second harmonic is used for the solid-state laser oscillator31.

A pulsed laser beam emitted from the semiconductor laser oscillator21and a pulsed laser beam emitted from the solid-state laser oscillator31are entered onto a semiconductor substrate50which is an annealing target, by way of a propagation optical system27. The pulsed laser beam emitted from the semiconductor laser oscillator21and the pulsed laser beam emitted from the solid-state laser oscillator are entered on the same area of the surface of the semiconductor substrate50.

Next, the configuration and the operation of the propagation optical system27will be described. The pulsed laser beam emitted from the semiconductor laser oscillator21is incident on the semiconductor substrate50by way of an attenuator22, a beam expander23, a homogenizer24, a dichroic mirror25, and a condensing lens26.

The pulsed laser beam emitted from the solid-state laser oscillator31is incident on the semiconductor substrate50by way of an attenuator32, a beam expander33, a homogenizer34, a bending mirror35, the dichroic mirror25, and the condensing lens26.

The beam expanders23and33collimate the pulsed laser beams having been incident thereon and expand the diameters of the beams. The homogenizers24and34and the condensing lens26shape a beam cross-section on the surface of the semiconductor substrate50into an elongated shape and uniformize light intensity distribution in the beam cross-section. The pulsed laser beam emitted from the semiconductor laser oscillator21and the pulsed laser beam emitted from the solid-state laser oscillator31are incident on substantially the same elongated area on the surface of the semiconductor substrate50.

The semiconductor substrate50is held on a stage41. An XYZ orthogonal coordinate system is defined in which a plane parallel to the surface of the semiconductor substrate50is set to be an X-Y plane and a direction normal to the surface of the semiconductor substrate50is set to be a Z direction. A control unit20controls the semiconductor laser oscillator21, the solid-state laser oscillator31, and the stage41. The stage41moves the semiconductor substrate50in an X direction and a Y direction in response to the control from the control unit20.

InFIG. 2A, a cross-sectional view of an insulated gate bipolar transistor (IGBT) as an example of a semiconductor device which is produced by the method according to the embodiment is shown. The IGBT is fabricated by forming an emitter and a gate on a surface on one side (hereinafter referred to as a “first surface”)50T of the semiconductor substrate50made of n-type silicon and forming a collector on a surface on the other side (hereinafter referred to as a “second surface”)50B. As the semiconductor substrate50, a silicon single crystal substrate is usually used. A structure of the surface on which the emitter and the gate are formed is made by the same process as a manufacturing process of a general MOSFET. For example, as shown inFIG. 2A, a p-type base region51, an n-type emitter region52, a gate electrode53, a gate insulating film54, and an emitter electrode55are disposed at a surface layer portion of the first surface50T of the semiconductor substrate50. It is possible to perform ON-OFF control of an electric current by a voltage between the gate and the emitter.

A p-type collector layer57and a low-concentration n-type buffer layer56are disposed at a surface layer portion of the second surface50B of the semiconductor substrate50. The buffer layer56is disposed at a position deeper than the collector layer57. The collector layer57and the buffer layer56are respectively formed by ion-implanting, for example, boron and phosphorus as dopants and performing activation annealing. The laser annealing apparatus shown inFIG. 1is applied to the activation annealing. A collector electrode58is formed on the surface of the collector layer57after the activation annealing.

The depth from the second surface50B to the interface between the collector layer57and the buffer layer56is, for example, about 0.3 μm. The depth from the second surface to the deepest position of the buffer layer56is within a range of 1 μm to 5 μm, for example.

InFIG. 2B, a cross-sectional view of the semiconductor substrate50at a step of performing laser annealing is shown. Boron is ion-implanted into a surface layer portion57aof the second surface50B of the semiconductor substrate50. Phosphorus is ion-implanted into a portion56adeeper than the surface layer portion57a. The boron in the surface layer portion57aand the phosphorus in the deep portion56ahave not been activated. Boron concentration of the surface layer portion57ais higher than phosphorus concentration of the deep portion56a. In this specification, the surface layer portion57ais referred to as a “high-concentration layer” and the deep portion56ais referred to as a “low-concentration layer”. Since the dose amount of boron is high, the high-concentration layer57ais in an amorphous state. A portion deeper than the interface between the high-concentration layer57aand the low-concentration layer56aremains in a single-crystal state. The device structure shown inFIG. 2Ahas been formed on the first surface50T of the semiconductor substrate50.

InFIG. 3A, laser pulse waveforms which are incident on the semiconductor substrate50(FIG. 2B) are schematically shown. InFIG. 3A, the pulse waveforms are shown in a rectangular shape. However, the actual pulse waveform includes portions such as rising, attenuation, and falling of a pulse. Emitting timings of the pulse waveforms shown inFIG. 3Aare determined by the control unit20(FIG. 1) controlling the semiconductor laser oscillator21and the solid-state laser oscillator31.

At time t1, incidence of a first laser pulse LP1emitted from the semiconductor laser oscillator21on the semiconductor substrate50is started. At time t2after the time t1, a second laser pulse LP2emitted from the solid-state laser oscillator31is incident on the semiconductor substrate50. Areas on which the first laser pulse LP1and the second laser pulse LP2are incident approximately overlap. The peak power of the second laser pulse LP2is higher than the peak power of the first laser pulse LP1and a pulse width PW2of the second laser pulse LP2is shorter than a pulse width PW1of the first laser pulse LP1. At time t3, the incidence of the second laser pulse LP2is ended. Thereafter, at time t4, the incidence of the first laser pulse LP1is ended. Alternatively, the second laser pulse LP2may be incident after the time t4.

The pulse width PW1of the first laser pulse LP1is greater than or equal to, for example, 10 μs. The pulse width PW2of the second laser pulse LP2is less than or equal to, for example, 1 μs. As an example, the pulse width PW1is within a range of 10 μs to 30 μs and the pulse width PW2is within a range of 100 ns to 200 ns. It is preferable to set the pulse width PW2of the second laser pulse LP2to be less than or equal to 1/10 of the pulse width PW1of the first laser pulse LP1.

InFIG. 3B, a plan view of an incident area of a laser pulse on the second surface50B of the semiconductor substrate50(FIG. 2B) is shown. The first laser pulse LP1(FIG. 3A) and the second laser pulse LP2(FIG. 3A) are incident on the same beam incident area40which is long in the X direction, on the second surface50B (FIG. 2B) of the semiconductor substrate50. For example, suitable length L and width Wt of the beam incident area40respectively are in a range of 2 mm to 4 mm and a range of 200 μm to 400 μm.

During annealing, the first laser pulse LP1and the second laser pulse LP2(FIG. 3A) are caused to be incident on the semiconductor substrate50at a certain repetition frequency while moving the semiconductor substrate50(FIG. 2B) in the Y direction. A distance that the semiconductor substrate50moves during one cycle of the repetition frequencies of the first laser pulse LP1and the second laser pulse LP2is represented by Wo. The beam incident areas40of the two first laser pulses LP1adjacent to each other on a time axis partially overlap each other. An overlap rate Wo/Wt of the two is, for example, 50%.

When the incidence of the first laser pulse LP1is started at the time t1shown inFIG. 3A, the temperature of the surface layer portion of the second surface50B (FIG. 2B) of the semiconductor substrate50begins to rise. At the point in time of the time t2, the temperature of the second surface50B of the semiconductor substrate50does not reach the melting point (in a range of 1300 K to 1430 K) of amorphous silicon. When the second laser pulse LP2is caused to be incident at the time t2, the temperature of the surface layer portion of the second surface50B of the semiconductor substrate50reaches the melting point of the amorphous silicon, and thus the surface layer portion is melted. The melted portion reaches the bottom of the high-concentration layer57a(FIG. 2B).

After the incidence of the second laser pulse LP2is ended, the temperature of the surface layer portion of the semiconductor substrate50is lowered and solidification occurs. At this time, a crystal epitaxially grows from the single-crystalline low-concentration layer56a(FIG. 2B), whereby the high-concentration layer57abecomes a single crystal. At the same time, the dopants implanted into the high-concentration layer57aare activated.

Also at the time t3or later, the incidence of the first laser pulse LP1(FIG. 3A) is continued, and therefore, a portion from the second surface50B of the semiconductor substrate50to the deep low-concentration layer56ais heated, and thus a temperature rises. In this way, the dopants implanted into the low-concentration layer56a(FIG. 2B) are activated. At the point in time when the incidence of the first laser pulse LP1is ended at the time t4, the temperature of the second surface50B of the semiconductor substrate50does not reach the melting point of single-crystal silicon. For this reason, the surface layer portion of the second surface50B of the semiconductor substrate50having been recrystallized is not re-melted.

In a case of causing the second laser pulse LP2to be incident later than the time t4, the dopants in the low-concentration layer56aare activate by the irradiation with the first laser pulse LP1. Thereafter, the high-concentration layer57a(FIG. 2B) is melted by the irradiation with the second laser pulse LP2. When the high-concentration layer57ais recrystallized, the dopants in the high-concentration layer57aare activated.

InFIG. 4, an example of dopant concentration distribution in a depth direction of the semiconductor substrate50is shown. A horizontal axis ofFIG. 4represents a depth from the second surface50B (FIG. 2B) of the semiconductor substrate50in a unit of “μm”. A vertical axis ofFIG. 4represents the concentration of added dopants in a unit of “cm−3” and by a logarithmic scale.

The concentration of boron (B) shows the maximum value at a depth of 0.3 μm from the second surface50B. Further, the concentration of phosphorus (P) shows the maximum value at a depth of 1.8 μm from the second surface50B. A foot (a tail) of the concentration distribution of phosphorus (P) extends to a depth of 3 μm from the second surface50B. A depth where the dopant concentration of boron and the dopant concentration of phosphorus become equal to each other is equivalent to the interface between the high-concentration layer57aand the low-concentration layer56a(FIG. 2B).

The dopants (phosphorus) added to a relatively deep portion, a depth of which exceeds, for example, 1 μm, at relatively low concentration are activated by the first laser pulse LP1(FIG. 3A) having a relatively long pulse width. The dopants (boron) added to a relatively shallow area, a depth of which is, for example, less than or equal to 1 μm, at relatively high concentration are activated by the second laser pulse LP2(FIG. 3A) having a relatively short pulse width. The second laser pulse LP2melts the semiconductor substrate50to a position deeper than a portion amorphized due to the ion implantation of the dopants at high concentration. The semiconductor substrate50in the incident area40(FIG. 3B) is melted by the irradiation with the second laser pulse LP2. When the melted area is solidified, liquid phase epitaxial growth occurs from a single-crystalline portion. In this way, the recrystallization of the amorphized portion and the activation of the dopants (boron) are performed.

InFIGS. 2A, 2B, and 4, an example is shown in which boron is implanted into the high-concentration layer57awhich is relatively shallow and phosphorus is implanted into the low-concentration layer56awhich is relatively deep. Also in a case of implanting other dopants, the same activation is performed. Further, dopants of the same conductivity type may be implanted into the high-concentration layer57aand the low-concentration layer56a. As an example, phosphorus may be implanted into both the high-concentration layer57aand the low-concentration layer56a. In this case, a high-concentration n-type layer is formed at a shallow portion and a low-concentration n-type layer is formed at a deep portion.

The activation of the dopants of the deep low-concentration layer56a(FIG. 2B) by the irradiation with the first laser pulse LP1(FIG. 3A) will be described with reference toFIGS. 5 and 6.

InFIG. 5, the simulation results of the relationship between an attainment temperature of a silicon wafer when the silicon wafer is irradiated with the first laser pulse LP1and a pulse width is shown. A horizontal axis represents the pulse width PW1of the first laser pulse LP1in a unit of “μs” and a vertical axis represents an attainment temperature in a unit of “K”. The wavelength of the first laser pulse LP1is set to be 915 nm. The irradiation condition of the first laser pulse LP1is set to be the condition that the surface temperature of the silicon wafer reaches the melting point (about 1690 K) of silicon. Solid lines a and b ofFIG. 5respectively show attainment temperatures at positions of depths of 3 μm and 100 μm from the surface of the silicon wafer.

It is found that if the pulse width is short, heat is not easily transmitted to a deep area. In order to activate the dopants at a depth position of 3 μm, it is preferable to set the pulse width to be greater than or equal to 5 μs. If the pulse width is too long, the temperature of a deep portion having a depth of about 100 μm, that is, the first surface50T (FIG. 2B) on which a device structure is formed is increased.

InFIG. 6, dopant concentration distribution in the depth direction and carrier concentration distribution after annealing are shown. A horizontal axis represents a depth from the surface of the silicon wafer in a unit of “μm” and a vertical axis represents concentration in a unit of “cm−3”. A solid line a shows the phosphorus concentration of when ion implantation of phosphorus is performed under the condition that acceleration energy is 2 MeV and a dose amount is 3×1013cm−2. This dopant concentration distribution corresponds to the dopant concentration distribution of the low-concentration layer56a(FIG. 2B). A solid line b shows carrier concentration after the first laser pulse LP1is emitted. The laser irradiation is performed under the condition that a pulse width is 20 μs and power density on an irradiated surface is 360 kW/cm2. From the results shown inFIG. 6, it is confirmed that about 70% or more of the implanted dopants is activated.

Next, the recovery of a portion amorphized by ion implantation will be described with reference toFIGS. 7A to 7C.

InFIG. 7A, a cross-sectional TEM image of a surface layer portion of the silicon wafer in which phosphorus is ion-implanted is shown. The ion implantation of phosphorus is performed under the condition that acceleration energy is 100 keV and a dose amount is 2×1015cm−2. The conditions of the ion implantation correspond to the ion implantation conditions of dopants into the high-concentration layer57a. InFIG. 7A, an amorphized portion is represented by a relatively light color.

InFIG. 7B, a cross-sectional TEM image of the surface layer portion of the silicon wafer after the irradiation with the second laser pulse LP2(FIG. 3A) is shown. As the second laser pulse LP2, the second harmonic of an Nd:YLF laser is used. The irradiation with the second laser pulse LP2is performed by a so-called double pulse method. The pulse width of one laser pulse is 130 ns, pulse energy density is 1.6 J/cm2, and delay time from the irradiation with the laser pulse of the first shot to the irradiation with the laser pulse of the second shot is 500 ns. In a portion melted by laser irradiation, epitaxial growth occurs from a single-crystal portion below the melted portion, and therefore, a good single crystal layer with few defects is obtained.

InFIG. 7C, a cross-sectional TEM image of the surface layer portion of the silicon wafer after the irradiation with the first laser pulse LP1(FIG. 3A) is shown. The wavelength of the first laser pulse LP1is 808 nm. The irradiation with the first laser pulse LP1is performed under the condition that a pulse width is 40 μs and power density is 300 kW/cm2. Under the irradiation conditions, the surface of the silicon wafer is not melted. If melting is made to a deep portion (the low-concentration layer56aofFIG. 2B) of the silicon wafer by a laser pulse, the dopant concentration distribution of the high-concentration layer57acollapses. For this reason, the irradiation with the first laser pulse LP1is performed under the condition that the silicon wafer is not melted. An annealing method of performing activation of dopants without melting a silicon wafer is referred to as “non-melting annealing”. In contrast, an annealing method of temporarily melting a silicon wafer is referred to as “melting annealing”. In the non-melting annealing, crystallization proceeds from the upper side and the lower side of an amorphized portion. A layer in an amorphous state remains approximately at the center with respect to a thickness direction. The crystal quality of a crystallized portion is also worse than the crystal quality of a recrystallized portion ofFIG. 7B.

As can be seen from the cross-sectional TEM images shown inFIGS. 7A to 7C, in the high-concentration layer57a(FIG. 2B) in an amorphous state, it is preferable to activate dopants by performing melting annealing by using the second laser pulse LP2(FIG. 3A) in which the pulse width is relatively short and the peak power is relatively high.

InFIG. 8A, an example of temporal waveforms of laser pulses when laser annealing is performed in the method according to the embodiment is shown. A horizontal axis represents elapsed time from rising of the first laser pulse LP1in a unit of “μs” and a vertical axis represents light intensity. The pulse width of the first laser pulse LP1is 15 μs. At the point in time when 14 μs has elapsed from the rising of the first laser pulse LP1, the second laser pulse LP2rises. The second laser pulse LP2is the second harmonic of an Nd:YLF laser and the pulse width thereof is 130 ns.

InFIG. 8B, the simulation results of a temperature change of the silicon wafer when the silicon wafer is irradiated with the first and second laser pulses LP1and LP2ofFIG. 8Aare shown. A horizontal axis represents elapsed time in a unit of “μs” and a vertical axis represents a temperature in a unit of “K”. Solid lines a to g inFIG. 8Brespectively show temperatures of the positions where a depth from the surface of the silicon wafer is 0 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, and 100 μm.

When the irradiation with the first laser pulse LP1is started, the temperature of the silicon wafer rises. At the point in time when 14 μs has elapsed after the irradiation is started, the surface temperature of the silicon wafer reaches 1660 K close to the melting point of silicon. At this point in time, the silicon wafer has not been melted. When the second laser pulse LP2is emitted, the surface temperature of the silicon wafer reaches the melting point, and thus the surface layer portion is melted.

InFIG. 8C, the simulation results of a temporal change of a melting depth of the silicon wafer are shown. A horizontal axis represents elapsed time in a unit of “μs” and a vertical axis represents a melting depth in a unit of “μm”. It is found that melting is made to a depth of about 0.3 μm due to the irradiation with the second laser pulse LP2. The dopants implanted into a portion shallower than the depth of 0.3 μm are activated due to the melting and the recrystallization of the surface layer portion.

As shown inFIG. 8B, at the point in time when about 14 μs has elapsed from the start of the irradiation with the first laser pulse LP1, the temperature of the depth position of 5 μm exceeds 1200 K. For this reason, the dopants implanted into a portion at a depth of about 5 μm, which has not been melted, are activated by solid-phase diffusion. In a case of performing prolonged annealing like annealing by an electric furnace, the activation of dopants begins at a temperature lower than 1200 K. However, in a case of performing heating by a laser pulse having a pulse width which is short in the order of a microsecond, if a temperature exceeds 1200 K, the activation of dopants begins to occur.

InFIG. 9, the relationship between elapsed time t2-t4from the falling time t4of the first laser pulse LP1to the rising time t2of the second laser pulse LP2and fluence of the second laser pulse LP2necessary for melting the silicon wafer is shown. The delay time from the time t4to the time t2is denoted by td. A horizontal axis represents the delay time td in a unit of “μs”. In a range where the delay time td is negative, the second laser pulse LP2rises before the falling time t4of the first laser pulse LP1. The left vertical axis ofFIG. 9represents the fluence of the second laser pulse LP2in a unit of “J/cm2”, and the right vertical axis represents the power of the second laser pulse LP2in a unit of “W”.

A solid line a shown inFIG. 9indicates the simulation results of the fluence of the second laser pulse LP2under the condition that the position at a depth of 0.3 μm of the silicon wafer is melted. A broken line b indicates the simulation results of the fluence of the second laser pulse LP2under the condition that the outermost surface of the silicon wafer is melted. The increase or decrease tendencies of the two when the delay time td changes are approximately equal to each other. A square symbol shown inFIG. 9indicates an actual measurement value of the power of the second laser pulse LP2when the outermost surface of the silicon wafer is melted. Since the area of the incident area40(FIG. 3B) and the pulse width of the first laser pulse LP1are constant, by multiplying the power of the second laser pulse LP2by a constant, it is possible to be converted into fluence. As shown inFIG. 9, the simulation results are sufficiently matched to the actual experimental results.

The origin (td=0) of the horizontal axis corresponds to the irradiation condition that the second laser pulse LP2rises simultaneously with the falling of the first laser pulse LP1.

The pulse width of the first laser pulse LP1is set to be 25 μs and the power density on the silicon wafer surface is set to be 310 kW/cm2. A position where the delay time td is −25 μs corresponds to the irradiation condition that the second laser pulse LP2rises simultaneously with the rising of the first laser pulse LP1.

If the delay time td is increased toward 0 μs from −25 μs (the rising time t2of the second laser pulse LP2is brought close to the falling time t4of the first laser pulse LP1), the fluence of the second laser pulse LP2necessary for melting the silicon wafer is reduced. This is because the silicon wafer has been preheated by the first laser pulse LP1at the point in time of the incidence of the second laser pulse LP2. In the vicinity of the delay time td=0, the fluence of the second laser pulse LP2necessary for melting the silicon wafer shows the minimum value.

The fluence of the second laser pulse LP2necessary for melting the silicon wafer increases as the delay time td becomes longer from 0. This is because the temperature of the silicon wafer is reduced (a preheating effect is reduced) after the falling time t4of the first laser pulse LP1.

As can be seen from the simulation results shown inFIG. 9and the experimental results, by adjusting the irradiation timings with the first laser pulse LP1and the second laser pulse LP2, it is possible to reduce the fluence of the second laser pulse LP2necessary for melting the silicon wafer. In other words, if the area of the incident area40(FIG. 3B) is constant, as the solid-state laser oscillator31(FIG. 1) for the second laser pulse LP2, it is possible to use a solid-state laser oscillator having small output. If the output of the solid-state laser oscillator31is constant, it is possible to increase the area of the incident area40.

InFIG. 10, the simulation results of an attainment temperature of the rear surface when a silicon wafer having a thickness of 100 μm is irradiated with the first laser pulse LP1and the second laser pulse LP2are shown. A horizontal axis represents the delay time td in a unit of “μs” and a vertical axis represents the attainment temperature of the rear surface in a unit of “K”.

The repetition frequencies of both the first laser pulse LP1and the second laser pulse LP2are set to be 2 kHz and the pulse widths of the first laser pulse LP1and the second laser pulse LP2are respectively set to be 25 μs and 0.15 μs. The power density of the first laser pulse LP1is set to be 310 kW/cm2. The fluence of the second laser pulse LP2is set to be the condition that a portion at a depth of 0.3 μm of the silicon wafer is melted.

The attainment temperature of the rear surface changes depending on the delay time td under the condition that the melting depth becomes a certain depth, 0.3 μm, on the laser irradiated surface. In a case of manufacturing the IGBT, it is not preferable that the attainment temperature of the rear surface become high. The attainment temperature of the rear surface shows the minimum value in the vicinity of the delay time td=0. If the delay time td is made longer than 0, the attainment temperature of the rear surface first shows the maximum value and then gradually decreases. However, as shown inFIG. 9, the fluence of the second laser pulse LP2has to be increased in order to melt the silicon wafer as the delay time td becomes longer. In order to suppress an increase in the fluence of the second laser pulse LP2, it is preferable that the delay time td be selected from a range which includes the minimum value in the vicinity of the origin. In a case where an allowable upper limit value TA of the attainment temperature of the rear surface is predetermined, inFIG. 10, it is favorable if the delay time td is set such that the attainment temperature of the rear surface becomes less than or equal to the allowable upper limit value TA.

Next, the preferable irradiation conditions for suppressing a rise in rear surface temperature will be described.

A lowest value TL and a highest value TH of the attainment temperature of the rear surface respectively appear in the vicinity of the delay time td of 0 and the vicinity of the delay time td of 75 μs. In order to obtain a sufficient effect of suppressing a rise in the attainment temperature of the rear surface by adjusting the incidence timings of the first laser pulse LP1and the second laser pulse LP2, it is preferable to set the attainment temperature of the rear surface to be less than or equal to an average value TM of the lowest value TL and the highest value TH.

In the simulation results shown inFIG. 10, the attainment temperature of the rear surface becomes less than or equal to the average value TM. between tf and tb of the delay time td. When the pulse width of the first laser pulse LP1is denoted by W, approximately, tf is −4W/5 and tb is W/4. That is, it is preferable to adjust the incidence timings of the first laser pulse LP1and the second laser pulse LP2such that the elapsed time t2-t4satisfies a relationship, −4W/5≦t2−t4≦W/4 . . . (1). If the pulse width W changes, the irradiation condition (the power density) is set such that a target attainment temperature of the substrate surface at the time of the falling of a pulse becomes substantially constant though the slope of a temperature rise in a period in which the first laser pulse LP1is emitted changes. For this reason, even in a case of performing activation annealing on the condition that the pulse width W is different, a lower limit value of t2-t4of the above-described conditional expression (1) can also be applied to the condition that the pulse width W is different.

After the falling of the first laser pulse LP1, that is, a range satisfying a relationship, 0≦t2−t4, the temperature of the substrate surface is reduced with time according to thermal diffusion, and thus a preheating effect is reduced. The thermal diffusion depends on a temperature profile in the depth direction of the substrate. For example, if the pulse width W becomes longer, a temperature rises even at a deeper portion and a temperature profile in the depth direction becomes gentle. When the temperature profile is gentle, a decrease in the surface temperature after the falling of the first laser pulse LP1also becomes gentle. That is, speed at which the preheating effect is reduced depends on the pulse width W, and if the pulse width W becomes longer, the preheating effect lasts a long time. In the simulation results shown inFIG. 10, the rear surface attainment temperature when the delay time td is W/2 is substantially equal to the rear surface attainment temperature when the delay time td is −25 μs, that is, when it is considered that there is no preheating effect. Therefore, it is considered that the preheating effect of the first laser pulse LP1is obtained by adjusting the timings of the first laser pulse LP1and the second laser pulse LP2so as to satisfy a relationship, t2−t4≦W/2.

Even if the condition such as the pulse width, the melting depth, or the thickness of the silicon wafer when the simulation ofFIG. 10is performed changes, the increase or decrease tendency of the graph shown inFIG. 10is maintained. Therefore, the above-described conditional expression (1) is not limited to the condition that the simulation is performed.

In the embodiment described above, if the wavelength of the first laser pulse LP1is set to be too short, laser energy is absorbed at a very shallow portion of the silicon wafer, and thus it becomes difficult to efficiently heat a deep portion. On the contrary, if the wavelength of the first laser pulse LP1is set to be too long, laser energy is not absorbed into the silicon wafer. In order to efficiently heat the low-concentration layer56a(FIG. 2B) of the silicon wafer by the first laser pulse LP1, it is preferable to set the wavelength thereof to be greater than or equal to 550 nm and less than or equal to 950 nm.

Light having a wavelength of 550 nm penetrates to a depth of about 1 μm of a silicon substrate. For this reason, if the wavelength of the first laser pulse LP1is set to be 550 nm, it is possible to efficiently heat a portion to a depth of 1 μm. A portion deeper than the depth of 1 μm is heated by heat conduction. If the wavelength of the first laser pulse LP1is set to be 650 nm, it is possible to efficiently heat a portion to a depth of about 3 μm. If the wavelength of the first laser pulse LP1is set to be 700 nm, it is possible to efficiently heat a portion to a depth of about 5 μm. In a case where the thickness of the silicon substrate is 100 μm, if the wavelength of the first laser pulse LP1is set to be longer than 950 nm, laser energy transmitting through the silicon substrate increases.

In order to activate the dopants of the low-concentration layer56a(FIG. 2B) which is deep, it is preferable to set a light penetration depth of the first laser pulse LP1into the semiconductor substrate50to be longer than a depth from the second surface50B of the semiconductor substrate50to the bottom of the low-concentration layer56a. Further, in order to reduce wasted light energy transmitting through the semiconductor substrate50, it is preferable to set the light penetration depth of the first laser pulse LP1to be shorter than the thickness of the semiconductor substrate50. The wavelength of the first laser pulse LP1is selected so as to satisfy the above-described light penetration depth. Here, the “light penetration depth” means a distance that light intensity is attenuated to 1/e.

It is preferable that the irradiation conditions with the first laser pulse LP1, specifically, the pulse width and the power density be set to be the conditions that the surface of the semiconductor substrate is not melted and the activation of the dopants in the low-concentration layer56abegins. Specifically, it is preferable to set the irradiation condition to be the condition that the temperature of the bottom of the low-concentration layer56aexceeds 1200 K.

It is preferable that the second laser pulse LP2include a green wavelength component which is easily absorbed into silicon, in order to efficiently melt the high-concentration layer57a(FIG. 2B) of the silicon wafer. In addition, because of the preheating effect of the first laser pulse LP1, light having a wavelength longer than that of green light is also easily absorbed into silicon. Therefore, the wavelength of the second laser pulse LP2may be set to be an infrared region having a wavelength of about 1 μm. Since light having a wavelength of an infrared region penetrates to a deep portion of the silicon substrate, the light melts the silicon substrate to a deeper portion, compared to a case of using light having a green wavelength region as the second laser pulse LP2.

In the embodiment described above, the manufacturing of the IGBT has been taken as an example. In addition, the method according to the above-described embodiment can be applied to the manufacturing of a semiconductor device in which it is desired to activate the dopants of a surface layer portion of the surface on one side of a semiconductor substrate and suppress a rise in temperature of the surface on the opposite side.

REFERENCE SIGNS LIST

20: control unit

21: semiconductor laser oscillator

27: propagation optical system

40: incident area of laser pulse

50T: first surface

50B: second surface

54: gate insulating film