Patent Publication Number: US-2023154756-A1

Title: Method for manufacturing semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of priority from Japanese Patent Application No. 2021-187690 filed on Nov. 18, 2021. The entire disclosure of the above application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a method for manufacturing a semiconductor device. 
     As a comparative example, there is a method for manufacturing a semiconductor device. In this manufacturing method, first, ions are implanted to a certain depth from a surface of a seed substrate, and an ion-implantation layer is formed. In the ion-implantation layer, an energy of the implanted ions weakens bonds between elements as compared to other semiconductor regions. Then, laser light is applied to the surface of the seed substrate to give energy to the implanted ions. Thereby, the bonds between elements are broken, and the semiconductor substrate is separated from the seed substrate along the ion-implantation layer. 
     SUMMARY 
     A method for manufacturing a semiconductor device includes: irradiating, with laser light, a semiconductor substrate having a p-type first semiconductor layer and an n-type second semiconductor layer so that the laser light converges on an interface between the first semiconductor layer and the second semiconductor layer, wherein each of the p-type first semiconductor layer and the n-type second semiconductor layer placed on the first semiconductor layer is formed of a compound semiconductor; and separating the semiconductor substrate into the first semiconductor layer and the second semiconductor layer along the interface. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    shows a manufacturing process of a semiconductor device according to a first embodiment. 
         FIG.  2    shows a manufacturing process of the semiconductor device according to the first embodiment. 
         FIG.  3    is a graph showing a result of simulating a relationship between an effective acceptor concentration of a first semiconductor layer and an effective donor concentration of a second semiconductor layer for applying a specific electric field intensity to a semiconductor substrate. 
         FIG.  4    shows a manufacturing process of the semiconductor device according to the first embodiment. 
         FIG.  5    is an enlarged view showing a vicinity of an interface in the manufacturing process of the semiconductor device according to the first embodiment. 
         FIG.  6    is a diagram for explaining a depletion layer formed in the vicinity of the interface between the first semiconductor layer and the second semiconductor layer. 
         FIG.  7    is a graph showing a relationship between a wavelength of light applied to gallium nitride, a photocurrent flowing through gallium nitride, and the electric field intensity applied to gallium nitride. 
         FIG.  8    shows a manufacturing process of the semiconductor device according to the first embodiment. 
         FIG.  9    shows a manufacturing process of the semiconductor device according to the second embodiment. 
         FIG.  10    shows a manufacturing process of the semiconductor device according to a modification. 
         FIG.  11    shows a manufacturing process of the semiconductor device according to the modification. 
     
    
    
     DETAILED DESCRIPTION 
     In the manufacturing method of the comparative example, a crystal structure of the ion-implantation layer is disturbed by the implanted ions. Therefore, when the seed substrate is separated along the ion-implantation layer, the crystal structure of the region exposed on the separation surface of the obtained semiconductor substrate is disturbed. Therefore, when this semiconductor substrate is used, for example, a difficulty arises in that the resistance of the semiconductor device increases. The present disclosure provides a technology capable of ensuring the quality of a separation surface when a semiconductor substrate is separated. 
     According to one example of the present disclosure, a method for manufacturing a semiconductor device includes: irradiating, with laser light, a semiconductor substrate having a p-type first semiconductor layer and an n-type second semiconductor layer so that the laser light converges on an interface between the first semiconductor layer and the second semiconductor layer, wherein each of the p-type first semiconductor layer and the n-type second semiconductor layer placed on the first semiconductor layer is formed of a compound semiconductor; and separating the semiconductor substrate into the first semiconductor layer and the second semiconductor layer along the interface. 
     In the above manufacturing method, the semiconductor substrate has the p-type first semiconductor layer and the n-type second semiconductor layer. Therefore, a depletion layer due to a built-in potential is formed in the vicinity of the interface between the first semiconductor layer and the second semiconductor layer. That is, the electric field is applied to the vicinity of the interface. Therefore, in the vicinity of the interface between the first semiconductor layer and the second semiconductor layer, Franz-Keldysh effect occurs, and long-wavelength (that is, low-energy) laser light is easily absorbed. That is, the light absorption efficiency is higher in a region in the vicinity of the interface between the first semiconductor layer and the second semiconductor layer than that in other semiconductor regions. Accordingly, by irradiating the interface with laser light so that the laser light converges on the interface, the laser light is efficiently absorbed in the vicinity of the interface, and the first semiconductor layer and the second semiconductor layer can be separated along the interface. Further, in this separation method, a crystal structure of the semiconductor on the separation plane is less likely to be disturbed as compared with a separation method of forming an ion-implantation layer. Hence, according to this manufacturing method, it may be possible to manufacture a high-quality semiconductor device. 
     The technical elements disclosed herein are listed below. The following technical elements are useful independently. 
     In one example of a manufacturing method in this specification, the compound semiconductor may be gallium nitride. When it is assumed that an effective acceptor concentration of the first semiconductor layer is Na (cm −3 ), an effective donor concentration of the second semiconductor layer is Nd (cm −3 ), an intrinsic carrier concentration of gallium nitride is n i  (cm −3 ), a dielectric constant of gallium nitride is ε GaN  (F/cm 2 ), a temperature is T (K), and a Boltzmann constant is k B  (J/K), the following expression may be satisfied. 
     
       
         
           
             
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     When the effective acceptor concentration of the first semiconductor layer and the effective donor concentration of the second semiconductor layer satisfy the above expression, an appropriate electric field is applied to the vicinity of the interface between the first semiconductor layer and the second semiconductor layer by the built-in potential, and it may be possible to efficiently absorb, in the vicinity of the interface, laser light having a wavelength longer than a wavelength corresponding to a bandgap of the gallium nitride. 
     According to a manufacturing method of one example of the present disclosure, in a process of applying the laser light, the laser light may be applied in a state where a voltage of the second semiconductor layer has a higher potential than that of the first semiconductor layer and is applied to the semiconductor substrate. 
     In such a configuration, the laser light is applied in a state where a reverse voltage is applied to the pn junction of the semiconductor substrate. By applying the reverse voltage to the pn junction, a depletion layer formed in the vicinity of the interface between the first semiconductor layer and the second semiconductor layer spreads. That is, the electric field applied to the vicinity of the interface is increased. Therefore, the Franz-Keldysh effect becomes greater, and longer-wavelength laser light can be absorbed in the vicinity of the interface. 
     First Embodiment 
     A manufacturing method of a semiconductor device according to a first embodiment will be described with reference to the drawings. This manufacturing method is characterized by a process of separating the semiconductor substrate into two layers. Accordingly, the manufacturing method of the present embodiment is not limited to semiconductor devices having a specific structure, and can be widely used for semiconductor devices having a semiconductor substrate including compound-semiconductors and semi-finished products thereof. Hereinafter, the process of separating the semiconductor substrate including a compound semiconductor into two layers will be mainly described, and the description of other manufacturing processes will be omitted. Note that, as for other manufacturing processes of the semiconductor device, necessary processes may be appropriately performed according to a structure of the semiconductor device. 
     First, as shown in  FIG.  1   , an n-type semiconductor layer  12  is prepared. The semiconductor layer  12  is made of gallium nitride. Note that the material of the semiconductor layer  12  is not limited to gallium nitride. The semiconductor layer  12  may be composed of, for example, other compound semiconductors such as silicon carbide. 
     Next, as shown in  FIG.  2   , a p-type region  14  is formed in a surface layer portion of the semiconductor layer  12  by ion-implanting, for example, magnesium as a p-type impurity into the semiconductor layer  12 . Then, an ultra-high-pressure annealing (UHPA) process is performed to activate the implanted magnesium ions, and the p-type first semiconductor layer  14  is formed. The remaining n-type region of the semiconductor layer  12  becomes a second semiconductor layer  16 . Thereby, a semiconductor substrate  20  having the second semiconductor layer  16  and the first semiconductor layer  14  placed on the second semiconductor layer  16  is obtained. 
     In a case where the semiconductor substrate  20  shown in  FIG.  2    is manufactured, the amount of implanted magnesium ions is adjusted so as to satisfy the following expression when it is assumed that the effective acceptor concentration of the first semiconductor layer  14  is Na (cm −3 ), the effective donor concentration of the second semiconductor layer  16  is Nd (cm −3 ), the intrinsic carrier concentration of gallium nitride is n i  (cm −3 ), the dielectric constant of gallium nitride is ε GaN  (F/cm 2 ), the temperature is T (K), and the Boltzmann constant is k B  (J/K). 
     
       
         
           
             
               
                 
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     The effective acceptor concentration of the first semiconductor layer  14  is calculated by subtracting the n-type impurity concentration from the p-type impurity concentration in the first semiconductor layer  14 . The effective donor concentration of the second semiconductor layer  16  is calculated by subtracting the p-type impurity concentration from the n-type impurity concentration in the second semiconductor layer  16 .  FIG.  3    shows a graph of the above expression. Although details will be described later, in the semiconductor substrate  20  that satisfies the above expression, the electric field intensity applied to the vicinity of an interface  18  between the first semiconductor layer  14  and the second semiconductor layer  16  is 1.2 MV/cm or more. 
     Next, as shown in  FIG.  4   , laser light  30  is applied so as to converge on the interface  18  between the first semiconductor layer  14  and the second semiconductor layer  16 . Here, scanning with the laser light  30  is performed along the interface  18  as indicated by an arrow  32  while the laser light  30  converges on the interface  18 . The wavelength of the laser light  30  is approximately 400 nanometers (nm). Further, a focal point depth d of the laser light  30  used here is extremely smaller than a thickness of the semiconductor substrate  20 , as shown in  FIG.  5   . Here, the laser light  30  is applied so that the interface  18  between the first semiconductor layer  14  and the second semiconductor layer  16  is within the focal point depth d. As the result, the laser light  30  is absorbed in the vicinity of the interface  18 , and high energy can be applied to the vicinity of the interface  18 . Thereby, crystal defects are formed along the interface  18 . 
     Normally, when light enters a semiconductor, the light is not absorbed by the semiconductor when the light energy is lower than the bandgap of the semiconductor. Since the gallium nitride has a bandgap of about 3.4 eV (corresponding to a wavelength of about 365 nm), normally, the laser light  30  having a wavelength of about 400 nm is hardly absorbed. However, when an electric field is applied to the semiconductor, a wave function of carriers existing in a conduction band and a valence band transitions, and an effective bandgap of the semiconductor becomes smaller. Therefore, the so-called Franz-Keldysh effect occurs, in which an absorption edge of light (that is, the minimum energy absorbed by the semiconductor) shifts toward the longer wavelength. 
     In the present embodiment, the first semiconductor layer  14  is the p-type and the second semiconductor layer  16  is the n-type. That is, a pn junction is formed between the first semiconductor layer  14  and the second semiconductor layer  16 . Accordingly, as shown in  FIG.  6   , in the vicinity of the interface  18  between the first semiconductor layer  14  and the second semiconductor layer  16 , a depletion layer  36  (that is an area indicated by dotted hatching) is formed due to the built-in potential. That is, the electric field is applied in the vicinity of the interface  18 , and the absorption edge of light shifts toward the long wavelength in the vicinity of the interface  18  due to the Franz-Keldysh effect. 
       FIG.  7    shows photocurrents flowing when light of various wavelengths is applied to gallium nitride having a bandgap of about 3.4 eV (corresponding to a wavelength of about 365 nm). A large photocurrent means that light is efficiently absorbed by gallium nitride. A horizontal axis of  FIG.  7    indicates the electric field intensity applied to the gallium nitride. As shown in  FIG.  7   , the electric field intensity of 1.2 MV/cm or more is necessary in order to obtain a photocurrent of 1 nA or more when gallium nitride is irradiated with light of 400 nm. That is, in order to shift the wavelength of light absorbed by gallium nitride from 365 nm to 400 nm, it is necessary to apply an electric field with an electric field intensity of 1.2 MV/cm or more to gallium nitride. When the wavelength of light absorbed by gallium nitride is shifted from 365 nm to 400 nm, the effective bandgap of gallium nitride decreases by about 10%. The graph of  FIG.  3    shows the result of simulating the relationship between the effective acceptor concentration of the first semiconductor layer  14  and the effective donor concentration of the second semiconductor layer  16  when the electric field intensity applied to the interface  18  is 1.2 MV/cm or more. As described above, in the present embodiment, the effective acceptor concentration of the first semiconductor layer  14 , the effective donor concentration of the second semiconductor layer  16 , and the like are adjusted so as to satisfy the relationship of the above expression representing a region  50  of  FIG.  3   . That is, in the vicinity of the interface  18  of the semiconductor substrate  20 , the light absorption edge transitions, and the effective bandgap of gallium nitride is reduced by about 10% or more. Therefore, in the present embodiment, the laser light  30  having the wavelength of approximately 400 nm can be efficiently absorbed in the vicinity of the interface  18 . Note that the laser light  30  is hardly absorbed in a region of the semiconductor substrate  20  to which no electric field is applied (that is, a region outside the depletion layer  36 ). Therefore, in the present embodiment, light is efficiently absorbed in the vicinity of the interface  18 , and little light is absorbed at a position away from the interface  18 . Accordingly, it may be possible to intensively form crystal defects in the vicinity of the interface  18 . 
     Next, as shown in  FIG.  8   , the first semiconductor layer  14  is separated from the second semiconductor layer  16 . For example, a support (e.g., tape, etc.) is attached to each of a surface of the first semiconductor layer  14  and a surface of the second semiconductor layer  16 , and both supports are pulled away from each other. Thereby, the first semiconductor layer  14  is separated from the second semiconductor layer  16 . Since the crystal defects are formed along the interface  18  in the laser irradiation process, the first semiconductor layer  14  can be separated from the second semiconductor layer  16  along the interface  18 . Thereafter, a thin semiconductor device can be manufactured using the separated first semiconductor layer  14  or second semiconductor layer  16 . 
     As described above, in this manufacturing method, in a laser irradiation process, it may be possible to form the crystal defects intensively at the interface  18  between the first semiconductor layer  14  and the second semiconductor layer  16 , and the crystal defects are hardly formed at positions other than the interface  18 . In particular, since the laser light  30  with which the semiconductor substrate  20  is irradiated has relatively low energy, it may be possible to reduce the influence of the laser light  30  on semiconductor regions other than the interface  18 . Accordingly, few crystal defects exist on the surfaces of the first semiconductor layer  14  and the second semiconductor layer  16  exposed after separation. That is, it may be possible to obtain a high-quality separation surface with few crystal defects. Accordingly, it may be possible to manufacture a high quality semiconductor device by using the separated first semiconductor layer  14  or the separated second semiconductor layer  16 . 
     Second Embodiment 
     Next, a manufacturing method according to a second embodiment will be described. In the second embodiment, after the semiconductor substrate  20  shown in  FIG.  2    of the first embodiment is manufactured, a power supply  44  is connected to the semiconductor substrate  20  as shown in  FIG.  9   . Specifically, an ohmic electrode  40  is formed on the surface of the first semiconductor layer  14 , and an ohmic electrode  42  is formed on the surface of the second semiconductor layer  16 . The power supply  44  is connected to each of the ohmic electrodes  40  and  42  in such a direction that a potential of the second semiconductor layer  16  is higher than that of the first semiconductor layer  14 . That is, the power supply  44  is connected in such a direction that a reverse voltage is applied to the pn junction of the interface  18 . 
     Next, while the reverse voltage is applied to the pn junction of the interface  18 , the laser light  30  is applied so as to converge on the interface  18  and the scanning is performed with the laser light  30  along the interface  18  in the same manner as in  FIG.  4   . Thereby, the crystal defects are formed along the interface  18 . Then, the semiconductor substrate  20  can be separated into the first semiconductor layer  14  and the second semiconductor layer  16  in the same manner as in  FIG.  8    of the first embodiment. 
     In the second embodiment, the laser light  30  is applied while the reverse voltage is applied to the pn junction of the interface  18  between the first semiconductor layer  14  and the second semiconductor layer  16 . By applying the reverse voltage to the pn junction, the depletion layer extending from the interface  18  toward each of the semiconductor layers  14  and  16  becomes larger. Therefore, the electric field intensity of the electric field applied to the vicinity of the interface  18  increases. That is, the Franz-Keldysh effect becomes greater, and the absorption edge of light shifts toward the longer wavelength. Therefore, in the second embodiment, it may be possible to absorb the laser light  30  having the longer wavelength (that is, lower energy) in the vicinity of the interface  18 . 
     In addition, in each of the above-described embodiments, the semiconductor substrate  20  may not satisfy the relationship of the first expression. In the first embodiment, as described above, since the interface  18  (pn junction) has no little depletion layer due to the built-in potential, the electric field is applied in the vicinity of the interface  18 . Therefore, even when the semiconductor substrate  20  does not satisfy the relationship of the first expression, the effective bandgap is small in the vicinity of the interface  18 , and light with a wavelength longer than about 365 nm is likely to be absorbed. Further, in the second embodiment, the reverse voltage is applied to the pn junction of the interface  18 . Therefore, it may be possible to apply a large electric field to the vicinity of the interface  18  even when the relationship of the first expression is not satisfied. 
     Further, in each of the above-described embodiments, the example, in which the semiconductor substrate  20  has the two laminated layers of the first semiconductor layer  14  and the second semiconductor layer  16  and the layers are separated, has been described. However, even in a case of a semiconductor substrate having three or more laminated layers, the technology in this specification can also be applied. For example, in the state shown in  FIG.  2    of the first embodiment, an n-type third semiconductor layer  22  may be formed on the semiconductor layer  14  as shown in  FIG.  10   . The third semiconductor layer  22  can be formed, for example, by epitaxially growing n-type gallium nitride. As shown in  FIG.  11   , the semiconductor substrate  120  obtained in such a manner is irradiated with a laser light  130  so that the laser light  30  converges on the interface  18  between the first semiconductor layer  14  and the second semiconductor layer  16 , and the scanning may be performed with the laser light  30  along the interface  18  as indicated by an arrow  132 . Here, the laser light  130  is applied from a position close to the second semiconductor layer  16 . Thereby, the laser light  130  is efficiently absorbed in the vicinity of the interface  18 , it may be possible to form the crystal defects intensively in the vicinity of the interface  18 . Note that the Franz-Keldysh effect can also occur at an interface  24  between the first semiconductor layer  14  and the third semiconductor layer  22 . However, since most of the laser light  130  is absorbed in the vicinity of the interface  18  (that is, most of the energy of the laser light  130  is consumed in the vicinity of the interface  18 ), the interface  24  is hardly affected. 
     Although the embodiments have been described in detail above, these are merely examples and do not limit the scope of claims. The technologies described in the claims include various modifications and modifications of the specific examples illustrated above. The technical elements described in the present specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technologies illustrated in the present specification or drawings achieve a plurality of objectives at the same time, and achieving one of the objectives itself has technical usefulness.