Patent Publication Number: US-2011067625-A1

Title: Crystal growth method and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Division of application Ser. No. 11/782,052 filed Jul. 24, 2007; the entire contents of which are incorporated herein by reference. 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-201369, filed on Jul. 24, 2006; the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a crystal growth method and apparatus. 
     2. Background Art 
     Metal-organic chemical vapor deposition (MOCVD) is used for crystal growth of compound semiconductors because of its good controllability on film thickness and composition. However, growth of nitride semiconductors such as GaN requires higher crystal growth temperature than that of AlGaAs-based and InAlGaP-based materials, hence causing various problems. 
     For example, in the so-called horizontal flow scheme where a wafer is held nearly horizontal and the flow direction of raw material gas and carrier gas is nearly parallel to the wafer, the gas is emitted from a nozzle located in the vicinity of the center of the crystal growth chamber. In this case, the gas temperature cannot be sufficiently decreased on the upstream side, resulting in thermal decomposition of the gas and its associated intermediate reaction. Consequently, the raw material element tends to run short in the vicinity of the wafer, and the semiconductor film growth rate often becomes lower than its maximum. 
     In particular, as a result of intermediate reaction associated with the decomposition of trimethylaluminum and ammonia, aluminum (Al) runs short on the downstream side. Thus, unfortunately, it is difficult to increase the Al composition ratio. 
     JP 2003-257867A discloses a technique for improving uniformity and controllability of crystal growth film across a wafer. This technique is directed to improving uniformity of electron mobility in AlGaAs-based or other materials by controlling the temperature of the region from the center of the susceptor to the wafer to be lower than that on the substrate. However, further improvement is needed for nitride materials because of their higher crystal growth temperature. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, there is provided a crystal growth method for forming a semiconductor film, the method including: while revolving one or more substrates about a rotation axis, passing raw material gas and carrier gas from the rotation axis side in a direction substantially parallel to a major surface of the substrate, the center of the substrate being located on a side nearer to the rotation axis than a position at which growth rate of the semiconductor film formed by thermal decomposition of the raw material gas is maximized. 
     According to another aspect of the invention, there is provided a crystal growth apparatus including: a rotary shaft; a susceptor having a substrate mounting portion interlocking with the rotary shaft, the susceptor revolving a substrate mounted on the substrate mounting portion about a rotation axis of the rotary shaft; a heater configured to heat the substrate mounted on the susceptor; a nozzle configured to emit raw material gas from the rotation axis side in a direction substantially parallel to a major surface of the substrate; and a first heat blocking part provided between the susceptor and the rotary shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view for illustrating a crystal growth method according to the embodiment of the invention.  FIG. 1A  is a schematic plan view of a susceptor, and  FIG. 1B  is a graph showing the dependence of the radial distribution of the growth rate. 
         FIG. 2  is a graph illustrating the supply ratio of Al raw material of the dependence of the Al composition ratio.  FIG. 2A  shows a comparative example, and  FIG. 2B  shows an embodiment of the invention. 
         FIG. 3  is a schematic cross-sectional view of a crystal growth apparatus according to the embodiment of the invention. 
         FIG. 4  is a schematic cross-sectional view of a crystal growth apparatus of the comparative example. 
         FIG. 5  shows a nozzle using a crystal growth apparatus of an embodiment of the invention.  FIG. 5A  is a schematic perspective view of the nozzle, and  FIG. 5B  is a schematic cross-sectional view thereof. 
         FIG. 6  shows a variation of the nozzle using a crystal growth apparatus of an embodiment of the invention.  FIG. 6A  is a schematic perspective view of the variation of nozzle, and  FIG. 6B  is a schematic cross-sectional view thereof. 
         FIG. 7  shows a second variation of the nozzle, using a crystal growth apparatus of an embodiment of the invention. 
         FIG. 7A  is a schematic perspective view thereof and  FIG. 7B  is a schematic cross-sectional view thereof. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the invention will now be described with reference to the drawings. 
       FIG. 1  is a schematic view for illustrating a crystal growth method according to the embodiment of the invention. 
     More specifically,  FIG. 1A  is a schematic plan view of a susceptor  12  holding a plurality of substrates  50  in a crystal growth apparatus, and  FIG. 1B  is a graph showing the radial distribution of the growth rate of semiconductor film from the center of the susceptor. 
     In a horizontal flow MOCVD apparatus for crystal growth of compound semiconductors, a susceptor  12  with a plurality of substrates  50  placed thereon is rotated about a rotation axis for improving uniformity of crystal growth film. More specifically, as illustrated in  FIG. 1A , a plurality of substrates  50  made of semiconductor material are mounted on the susceptor  12 . The susceptor  12  rotates in the direction of arrow K (or opposite thereto) about point P, the rotation center. Hence each substrate  50  revolves about the point P. 
     On the other hand, the nozzle  42  is fixed. Raw material gas and carrier gas are issued out in the radial direction R and nearly parallel to the major surface of the substrate  50  from the outlet of the nozzle  42  located at a radial distance RN. In  FIG. 1A , gas flow in only one direction is shown. 
     By the revolution of the substrate  50 , the circumferential distribution of the gas flow from the nozzle  42  and of the temperature of the susceptor  12  can be averaged, and the circumferential variation of film thickness and characteristics can be reduced. In order to achieve uniform thickness and characteristics among semiconductor films formed by crystal growth on the plurality of substrates  50 , the centers of the substrates  50  are radially located nearly equidistant from the point P, that is, at a distance RW. Furthermore, by rotating each of the substrates  50  on its axis in the direction of arrow G (or opposite thereto), the film thickness and characteristics can be made uniform across the substrate. 
     When the substrates  50  are heated by a heater, the raw material gas supplied by the horizontal flow is thermally decomposed, and semiconductor film is grown above the crystal growth temperature. Here, the growth rate of the semiconductor film is substantially determined by temperature distribution in the susceptor  12  and the distribution of the supplied amount of the raw material gas. 
     In  FIG. 1B , the solid line representing the growth rate includes regions A and B. The region A is a region on the near side of position RG at which the growth rate is maximized, near to the rotation axis, where the growth rate increases with the increase of radial distance R. The region B is a region outside the position RG, where the growth rate decreases with the increase of distance R. Here, the region A represents a region where the supply of growth raw material increases with the radial distance, and the region B represents a region where the supply of growth raw material decreases with the radial distance. The growth rate increases substantially linearly in the region M of R 1 ≦R≦R 2 , is saturated to its maximum at RG in the region Q, and decreases substantially linearly in the region N. On the rotation axis side of the region M and outside the region N, the growth rate decreases relatively steeply. In this embodiment, crystal growth is performed so that the center RW of the substrate  50  is located on the rotation axis side of the position RG at which the growth rate of semiconductor film is maximized. 
     Conversely, if crystal growth is performed so that the center RW of the substrate  50  is located outside the position RG of maximum growth rate, deposits of growth film deposited on the susceptor  12  and inside the crystal growth apparatus increase on the gas upstream side of the substrate  50 , that is, near the rotation axis of the susceptor  12 . If the deposits are decomposed or detached and reattached to the substrate  50 , pitting and roughness occur on the surface of the growth film and degrades the quality and characteristics of the growth film. 
     In contrast, in this embodiment, the center RW of the substrate  50  is located on the side nearer to the rotation axis than the position RG. Thus deposits inside the crystal growth apparatus on the gas upstream side of the substrate  50  can be decreased. Hence pitting and roughness can be reduced, and the quality and characteristics of growth film can be improved. 
     Furthermore, when the substrate  50  is rotated on its axis with its near entirety located in the region M illustrated in  FIG. 1B , the radial variation of growth rate can be averaged to further enhance the uniformity of growth film. That is, because the growth rate has a nearly linear distribution in the region M, the distribution of thickness and characteristics of growth film can be made uniform by rotating the substrate  50  on its axis in the region M. 
     As a method for crystal growth by placing the center of the substrate  50  within the region A of the growth rate curve illustrated in  FIG. 1B , it is possible to use the method of increasing the total flow rate of raw material gas. Then the region A can be extended to shift the position RG of maximum growth rate to the downstream side of gas flow. 
     It is also possible to use the method of decreasing the temperature on the upstream side. By decreasing the temperature on the upstream side, thermal decomposition of raw material gas can be reduced on the upstream side. Hence it is possible to shift the position RG of maximum growth rate to the gas downstream side, thereby extending the region A. 
     In addition to the effect of improving the quality and uniformity of growth film, this embodiment can also achieve the effects of reducing raw material gas, increasing mixed crystal ratio, and increasing dopant concentration. 
     That is, the center RW of the substrate  50  is located on the side nearer to the rotation axis than the position RG. Thus thermal decomposition and intermediate reaction of raw material gas can be reduced on the upstream side of the substrate. Hence attachment of deposits to this portion can be prevented, and waste of raw material gas can be reduced. 
     Next, the effect of achieving higher aluminum (Al) composition ratio is illustrated with reference to nitride semiconductor materials. 
       FIG. 2  is a graph illustrating the dependence of the Al composition ratio of growth film on the supply ratio of Al raw material. More specifically,  FIG. 2A  shows a comparative example of crystal growth in the region B where the growth rate decreases with radial distance R, and  FIG. 2B  shows an example of growth in the region A illustrated in  FIG. 1B . 
     In the comparative example, because violent intermediate reaction of Al raw material with other raw materials occurs before deposition species reach the substrate, the Al element is depleted in the vicinity of the substrate. Thus the Al composition ratio is not proportional to the Al raw material supply ratio, but saturated. This tendency is pronounced presumably because Al is more reactive than Ga, for example. If the Al raw material supply ratio is further increased, the Al composition ratio rather decreases. The Al composition ratio decreases also when the pressure in the chamber is increased. 
     In contrast, growth in the region A as illustrated in  FIG. 1B  significantly reduces the region where intermediate reaction occurs on the upstream side of the substrate. That is, because the Al raw material element is not depleted, the Al composition ratio of growth film grown on the substrate can be increased with the increase of the Al raw material supply ratio. Consequently, the Al composition ratio can be further increased in InGaAlN and other materials. Here, for preventing intermediate reaction, it is more effective to decrease the temperature in the vicinity of the upstream portion to generally 500° C. or less. The material is not limited to nitride semiconductor materials, but a similar effect is also achieved for InGaAlP-based and AlGaAs-based materials. 
     Next, again with reference to nitride semiconductor materials, a description is given of InGaAlN or other material being doped with magnesium (Mg), a p-type dopant. 
     In the case of crystal growth in the region B where the growth rate decreases in the radial direction R, as with the case described above with reference to  FIG. 2 , Mg concentration in the growth film is not proportional to the flow rate of Mg raw material such as Cp 2 Mg, but saturated. This is presumably because Mg is consumed and depleted by intermediate reaction on the upstream side of the substrate. The Mg concentration significantly decreases also when the pressure in the chamber is increased. 
     In contrast, growth in the region A illustrated in  FIG. 1B  can significantly reduce the region where intermediate reaction occurs on the upstream side of the substrate. Consequently, because Mg is not depleted, the Mg concentration in the growth film can be increased with the increase of Mg raw material gas flow rate. Hence the Mg concentration can be further increased in InGaAlN or other material. 
     In this embodiment, when the temperature on the upstream side of the substrate is decreased to steeply decrease the growth rate in the region S of RN&lt;R&lt;RD in the region A shown in  FIG. 1B , the effects described above can be further enhanced. That is, by decreasing the temperature on the upstream side of the substrate, decomposition and intermediate reaction of raw material gas in that portion can be further reduced. Consequently, it is possible to further enhance the above effects of improving the quality of growth film, reducing raw material gas, improving mixed crystal ratio, and increasing dopant concentration. 
     Next, a crystal growth apparatus for realizing the crystal growth method according to this embodiment is described. 
       FIG. 3  is a schematic cross-sectional view of a crystal growth apparatus according to the embodiment of the invention. In this figure, the same elements as those described above with reference to  FIG. 1  are marked with like reference numerals. 
     A vacuum chamber  60  includes a nozzle  42  made of SUS having a channel for cooling water  46 , an annular counterplate  40  made of quartz surrounding the lower portion of the nozzle  42 , a heater  24 , a heater cover  25  made of carbon, and a reflector  26 . The vacuum chamber  60  further includes an exhaust port  44  for exhausting raw material gas and carrier gas remaining after reaction. The nozzle  42  has a plurality of outlets for issuing group-III raw material gas, group-V raw material gas, carrier gas, or mixed gas thereof. The shape of the nozzle  42  will be described later with reference to examples. 
     A rotary shaft  10  is provided above the nozzle  42 , and a disk-shaped susceptor  12  is attached to the rotary shaft  10 . The susceptor  12  is made of carbon covered with silicon carbide (SIC) and includes substrate mounting portions, each having an opening adapted to the shape of substrate. Carbon is suitable as susceptor material because it has good workability and wide temperature range up to 3000° C. The susceptor  12  is coupled to the rotary shaft  10  via a heat insulating member (first heat blocking part)  20  and a heat conducting member  22 , and rotated while interlocking with these members. The heat insulating member  20  is made of quartz having low thermal conductivity. On the other hand, the heat conducting member  22  has higher thermal conductivity than carbon forming the susceptor, and is made of a compound such as boron nitride (BN) or a metal such as inconel. 
     A flow channel (not shown) for passing gas or liquid coolant  48  is suitably provided inside the rotary shaft  10 . The rotation axis AA′, which is the center line of the rotary shaft  10 , substantially matches the center line of the nozzle  42 . 
     By rotating the susceptor  12  about the rotation axis AA′, circumferential variation of the distribution of gas flow and/or temperature, if any, can be made uniform. Furthermore, the substrate  50  is rotated on its axis about BB&#39; (see  FIG. 3 ) as indicated by arrow G in  FIG. 1A  for averaging the variation of growth rate in the radial direction R to form uniform film. To this end, for example, it is suitable to rotate a substrate holder  16  made of BN by gears (not shown). Moreover, a heat spreader  28  made of SIC-coated carbon can be placed above the substrate  50  with a gap of e.g. less than 1 mm to make the substrate temperature more uniform. Here, as illustrated in  FIG. 3 , the substrate  50  can be placed face-down to prevent dust and/or foreign matter from falling on the surface of the substrate  50  in the vacuum chamber  60 . 
     In the crystal growth apparatus of this example, the temperature increase in the vicinity of the nozzle  42  can be restricted. More specifically, the heat conducting member  22  is attached to the rotary shaft  10  provided above the nozzle  42 . The heat insulating member  20  is provided between the heat conducting member  22  and the substrate mounting portion. Consequently, heat transfer from the substrate holder  16  to the heat conducting member  22  can be blocked. Furthermore, the rotary shaft  10  is cooled by the coolant  48 , and hence the heat conducting member  22  provided at the tip of the rotary shaft  10  can also be kept at a low temperature. Moreover, thermal conduction by gas and radiation indirectly cool the nozzle  42  via the heat conducting member  22 . Thus the temperature profile in the region from the outlet of the nozzle  42  to the substrate  50  can be made steep. Consequently, thermal decomposition and intermediate reaction of raw material gas on the upstream side of the substrate  50  can be effectively prevented. That is, the growth rate can be steeply decreased from the position RD toward the position RN, as in the region S shown in  FIG. 1B . 
     The heat insulating effect of the heat insulating member  20  can be enhanced by increasing its length along the radial direction R. Hence its shape can be suitably determined by taking into consideration the outer diameter of the susceptor  12 . Because the nozzle  42  can be cooled by cooling water  46  and carrier gas, the cooling effect in the region on the upstream side of the substrate can be further increased by decreasing the spacing between the nozzle  42  and the susceptor  12  to e.g. about 1 mm. Here, the cooling systems for the nozzle  42  and for the portion below the counterplate  40  can be made independent, and temperature control can be performed separately in each region. 
     On the other hand, the heater  24  needs to have high heat generation for heating the substrate  50  to the crystal growth temperature. For blocking the resulting radiant heat, it is more preferable to place the reflector (second heat blocking part)  26  between the heater  24 , and the rotary shaft  10  and the heat conducting member  22 . The reflector  26  can be formed by coating an alloy material such as inconel with a material with low thermal conductivity and further providing a gold plating layer on the surface thereof. The gold plating layer is preferable because it has a reflectivity of as high as 98.2% to infrared radiation at a wavelength of 1000 nm and chemically stable at high temperatures. 
       FIG. 4  is a schematic cross-sectional view of a crystal growth apparatus of the comparative example. The same elements as those in  FIG. 3  are marked with like reference numerals and not described in detail. 
     Also in this comparative example, the vacuum chamber  60  includes a susceptor  13  made of SiC-coated carbon, which has substrate holders  16  for attaching substrates  50  and is rotatably coupled to the rotary shaft  10 . However, this comparative example does not include the heat insulating member  20 , the heat conducting member  22 , and the reflector  26  described above with reference to  FIG. 3 . 
     In this comparative example, heat emitted from the heater  24  is conducted to the center of the susceptor  13  made of carbon and increases temperature in the vicinity of the gas nozzle  43 . Temperature at the center of the rotary shaft  10  and the susceptor  13  is increased also by directly receiving radiant heat from the heater  24 . 
     Consequently, temperature increases in the region between RN ( FIG. 1B ), where the outlet of the nozzle  43  is located, and the end of the substrate  50  on the center line AA′ side. In this region, organometallic gas containing group-III element and ammonia gas containing group-V element start decomposition and intermediate reaction. This shifts the position RG of maximum crystal growth rate close to the rotation axis (center line AA′). 
     In contrast, in the example shown in  FIG. 3 , by restricting temperature increase in the portion on the upstream side of the substrate  50 , thermal decomposition and intermediate reaction of raw material gas is prevented in this portion, and the position RG of maximum crystal growth rate can be shifted away from the rotation axis (center line AA′). Consequently, the center RW of the substrate  50  can be located on the side nearer to the rotation axis than the position RG. Thus, as described above, it is possible to achieve the effects of improving the quality of growth film, reducing raw material gas, improving mixed crystal ratio, and increasing dopant concentration. 
     Next, returning to  FIG. 3 , the cooling structure in the vicinity of the counterplate  40  is described. The fixed part of the crystal growth apparatus including the nozzle  42  is cooled by cooling water  46 . If the counterplate  40  opposed to the susceptor  12  is excessively cooled, difference from the growth temperature increases, thereby increasing deposits on the counterplate  40  and depleting ammonia reaction species. 
     Hence, it is preferable to provide a gap  72  or heat insulator below the counterplate  40  or to interpose an annular molybdenum plate  74  having a suitable length along the radial direction R, thereby reducing deposits on the counterplate  40  and preventing depletion of ammonia reaction species. 
     Next, the nozzle  42  operable with the crystal growth apparatus of this embodiment is described. 
       FIG. 5A  is a schematic perspective view of the nozzle  42 , and  FIG. 5B  is a schematic cross-sectional view thereof. 
     The nozzle  42  includes channels for carrier gas, for mixed gas of group-III gas and carrier gas, and for mixed gas of group-V gas and carrier gas, and has respective outlets at the position R=RN. The nozzle  42  further includes a flow channel for cooling water  46  as illustrated in  FIG. 3 . The gap D between the upper face of the nozzle  42  and the lower face of the heat conducting member  22  can be set to as small as e.g. 1 mm to further decrease temperature in the vicinity of the nozzle outlets with the help of radiation and thermal conduction to the heat conducting member  22 . 
     The heat conducting member  22  is further cooled by emitting carrier gas  61  upward from the gas channel along the center line AA′ of the nozzle  42 . Furthermore, the upper portion of the nozzle  42 , in the closest proximity of the susceptor  12 , has vertically cut slits  70  at nearly equiangular spacing, through which mixed gas of group-V gas, which is ammonia, and carrier gas are issued laterally. Moreover, by reducing the gap D between the upper face of the nozzle  42  and the lower face of the heat conducting member  22 , the flow rate of issued group-V gas can be adequately controlled, and the group-V and group-III raw material gases can be mixed more uniformly. Issuing ammonia, which is the group-V gas, from the upper side facilitates decomposition of ammonia, allowing ammonia reaction species to easily reach the substrate. 
       FIG. 6  shows a variation of the nozzle  43 , where  FIG. 6A  is a schematic perspective view thereof and  FIG. 6B  is a schematic cross-sectional view thereof. 
     This variation includes no slits  70 . It is also possible to use this type of nozzle  43 . 
       FIG. 7  shows a second variation of the nozzle  42 , where  FIG. 7A  is a schematic perspective view thereof and  FIG. 7B  is a schematic cross-sectional view thereof. 
     In this variation, the slit  70  has an opening directed to the lateral side of the nozzle  42 . Thus group-V gas is not directly in contact with the heat conducting member  22  being at high temperature. Hence decomposition of raw material gas can be reduced. Furthermore, by issuing hydrogen or other carrier gas  60  upward from the nozzle  42  toward the heat conducting member  22 , it is possible to further cool the heat conducting member  22 . 
     As described above, in the crystal growth method and apparatus of this embodiment, growth is performed primarily in the region A of increasing growth rate. Consequently, deposits in the chamber can be reduced, and degradation in the quality and characteristics of growth film due to reattachment of deposits can be prevented. Furthermore, by preventing decomposition and intermediate reaction of raw material gas on the upstream side of the substrate, depletion of the raw material element can be prevented, and thereby it is possible to obtain Al mixed crystal with increased Al composition ratio and nitride semiconductor devices with increased Mg doping concentration. Moreover, thermal decomposition of raw material gas not contributing to crystal growth can be reduced, and hence utilization efficiency of materials can be improved. 
     The embodiment of the invention has been described with reference to examples. However, the invention is not limited to these examples. For instance, the susceptor, substrate holder, counterplate, reflector, heat insulating member, heat conducting member, heat spreader, and other elements constituting the crystal growth apparatus that are variously adapted by those skilled in the art are also encompassed within the scope of the invention as long as they include the features of the invention.