METHOD OF FORMING SINGLE CRYSTAL SILICON USING SPE SEED AND LASER CRYSTALLIZATION

Abstract: A method of forming a single crystal silicon film using seeded laser crystallization. The method includes forming an insulating layer on a substrate, with seed windows formed in the insulating layer so as to expose portions of the substrate. A layer of amorphous silicon is formed on the insulating layer and in the seed windows. The amorphous silicon layer is heated so as to produce solid phase epitaxial growth of a single crystal silicon film in the seed window and extending over a portion of the insulating layer. During this heating step, the remainder of the amorphous silicon layer is converted to a polysilicon film. A laser beam is scanned across the polysilicon film to extend the single crystal silicon film over the remaining portion of the insulating layer by performing seeded laser crystallization using the solid phase epitaxial single crystal silicon film as a seed.

Description: METHOD OP FORMING SINGLE CRYSTAL SILICON USING SPE SEED AND LASER CRYSTALLIZATION BACKGROUND OP THE INVENTION 1. Field of the Invention This invention relates to a method of forming a sinqle crystal silicon film on an insulator, and particularly to a method in which seeded laser crystallization is employed. 2. Description of the Related Art Many semiconductor devices employ a structure including a silicon crystal on an insulator (SOI). Laser crystallization is one conventional method by which a sinqle crystal silicon film is grown on an insulator. Referring to PIG. 1, an insulating layer 20 of silicon dioxide (Si0 2 ) is formed on a silicon substrate 22, and a window 24 is formed in the insulatinq layer 20. Then, a polysilicon layer 26 is formed on the insulating layer 20 and on the portion of the silicon substrate 22 which is exposed by the window 24. Laser crystallization is performed in order to convert the polysilicon layer 26 to a single crystal, by scanning a laser beam 28 startinq with a seed area at the interface of the polysilicon layer 26 and the silicon suhstrate 22 in the window 24. The laser beam 2ft is scanned across the polysilicon layer 26 to heat the polysilicon layer 26 to the melting point so as to crystallize the polysilicon layer 26. However, the conduction properties of the structure illustrated in FIG. 1, cause problems when laser crystallization is performed. In particular, heat conducts through the window 24 more rapidly than it conducts through the silicon dioxide insulating layer 20. In laser crystallization, it is desirable to melt all of the polysilicon at the same time. However, with the structure of FIG. 1, the portion of the polysilicon layer 26 over the silicon dioxide insulating layer 20 will heat up faster than the seed area (i.e., the portion of the polysilicon layer 26 in the window 24) because of the fact that the heat conducts through the window 24 more rapidly than it conducts through the oxide insulating layer 20. This makes it difficult to melt all of the polysilicon layer 26 at the same time. In addition, the portion of the polysilicon layer 26 in the window 24 will start to solidify more rapidly than the polysilicon over the insulating oxide layer 20. FIG. 2 is a graph of laser power versus temperature for the portion of the polysilicon layer 26 over the seed area and the portion of the polysilicon layer 26 over the oxide insulating layer 20. As illustrated in FIG. 2, the temperature in the portion of the polysilicon layer 26 over the oxide insulating layer 20 rises with power until the melting point of silicon (1420 β C) is reached. When polysilicon melts its reflectivity doubles, so not all of the film melts simultaneously; rather, as power is increased, an increasing portion of the film melts without a rise in temperature until all of the film is melted. Thus, the temperature of the polysilicon layer 26 over the oxide insulating layer 20 levels off at the melting point of silicon and does not increase until a higher power level which is sufficiently high to melt the entire film with the increased reflectivity, is reached. Finally, at the point marked MAX on the qraph in FIG. 2, the portion of the polysilicon layer 26 over the oxide insulatinq layer 20 will beqin to agqlomerate, so that balls of silicon and holes will be created in the silicon layer. When the thickness of the oxide insulatinq layer 20 is increased, this becomes a greater problem. It is also possible that the silicon under the oxide insulating layer 20 could melt, in which case the silicon dioxide would not be anchored and could rise up and thin out the upper polysilicon layer. When the thickness of the oxide insulating layer 20 is reduced, this becomes a greater problem. Thus, the ranqe of laser power for scanninq the portion of the polysilicon layer 26 over the oxide insulatinq layer 20 must be limited to be below the point marked MAX on the qraph in PIG. 2. In contrast to the portion of the polysilicon layer 26 formed on the oxide insulatinq layer 20, the portion of the polysilicon layer 26 over the seed area will have a lower temperature for the same power absorption. Thus, the laser power marked MIN on the graph in FIG. 2 is required in order to reach the melting point of silicon for the portion of the polysilicon layer 26 over the seed area. As a result of the different heat conductive properties explained above, the performance of laser crystallization over a seed area and an oxide insulating layer, is made very difficult, since the laser power must be maintained between the points marked MIN and MAX on the graph of FIG. 2. This ranqe, which is a so-called "laser power window", will vary with the thickness of the oxide insulatinq layer. The laser power window can be extremely narrow and the required uniformity in the crystallization parameters may be difficult if not impossible to obtain. For example, typically a power window overheatinq problem will be caused if the thickness of the oxide insulating layer 20 is over 5000A, because the power window may be in the range of a fraction of a percent. FIG. 3 is an additional graph for illustrating the laser power ranges over the oxide insulatinq layer 20 and over the seed area. As illustrated in PIG. 3, it is only in the overlapping area between the minimum power for the seed area and the maximum power for the oxide insulating layer 20 (i.e., the "laser power window") which is available for performing laser crystallization. As indicated above, if the laser power extends outside the laser power window, insufficient crystallization or overheatinq (causinq warpaqe of the substrate, agglomeration, etc.) can occur. As an example, in the case of a 0.2 micron thick insulating silicon dioxide film, the power window is several percent when the substrate temperature is 500 β C. as described in "Device Performances of a Submicron SOI Technology", A.J. Auberton Herve et al., Proc. of 1984, IEDM Meeting, San Francisco, pp. 808-811. A 0.2 micron oxide insulator is too thin to provide high quality SOI circuit isolation with acceptable yield. Also, a power window of only several percent is difficult to maintain durinq laser crystallization, and does not allow for variations in film thicknesses normally present in production processinq. Thus, there is a need in the art for a laser crystallization method which is not limited by a narrow laser power window. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of forminq a sinqle crystal silicon film on an insulator which overcomes the deficiencies of prior art methods. In particular, it is an object of the present invention to provide a laser crystallization method which is not limited to thin insulating layers. A further object of the present invention is to provide a method of forming a single crystal silicon film on an insulator, so that a hiqh quality isolated SOI circuit can be formed thereon. In the method of the present invention, an insulating layer is formed on a substrate and a seed window is formed in the insulating layer. A layer of amorphous silicon is formed on the insulating layer and on the portion of the substrate in the seed window. Then, the amorphous silicon layer is heated so as to cause solid phase eDitaxial qrowth of a solid phase epitaxial sinqle crystal " silicon film in the seed window and extendinq over a portion of the insulating layer. During the heating step, the remainder of the amorphous silicon layer is converted to a polysilicon film. Next, a laser is scanned across the polysilicon film to extend the single crystal silicon film over the remaining portion of the insulating layer by performing seeded laser crystallization using the solid phase epitaxial sinqle crystal silicon film as a seed. The method of the present invention provides siqnificant advantaqes over the prior art in that the laser power window is extended to be as wide as the laser power window for laser crystallization over the insulating layer (see FIGS. 2 and 3). In addition, as a result of the improved laser power window, the thickness of the insulating layer can be increased, so that an SOI circuit with hiqh quality isolation properties can be formed thereon. These toqether with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanyinq drawings forming a part hereof, wherein like numerals refer to like parts throughout. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a semiconductor structure for describing a prior art method of laser crystallization; FIG. 2 is a graph of laser power versus temperature for both polysilicon over an insulatinq oxide layer and polysilicon over a seed area; FIG. 3 is a qraph, similar to FIG. 2, for illustrating the "laser power window" for the prior art method of laser >.-ystallization described with respect to FIG. 1; and FIGS. 4-10 are cross-sectional views for describing the steps of the method of forming a sinqle crystal silicon film in accordance with an embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referrinq to FIG. 4, in the method of the present invention, an insulating layer 30 is formed on a silicon substrate 32 and seed windows 34 are formed to expose portions of the silicon substrate 32. In the preferred embodiment a LOCOS (local oxidation of silicon) oxide film approximately 1 micron thick is qrown, as the insulatinq layer 30, over the portion of the substrate 32 where the final SOI film will be located. As a result, the seed windows 34 are formed in the insulating layer 30. Referring to FIG. 5, an amorphous silicon layer 36 is deposited on the insulating layer 30 and on the portions of the silicon substrate 32 in the seed windows 34. In the preferred embodiment the amorphous silicon layer 36 is approximately 0.2 to 0.5 microns thick. Next, the amorphous silicon layer 36 is heated so as to cause solid phase epitaxial (SPE) growth of a solid phase epitaxial single crystal silicon film 38 in the seed windows 34 and extendinq laterally over a portion of the insulating layer 30 (FIG. 6). During this heating step, the remaining portion of the amorphous silicon layer 36 will be converted to a polysilicon film 40. In the preferred embodiment, the solid phase epitaxial crystallization takes place in an oven by isothermal heating of the substrate to a temperature in the range of 560 to 600 β C (if higher temperatures are employed, then nucleation will occur predominantly in the portions of the amorphous silicon layer 36 over the insulating layer 30; if lower temperatures are used to avoid nucleation on the insulator and to grow the SPE crystallized silicon further, then the crystallization process becomes too time- consuming). In the preferred embodiment, the SPE single crystal silicon film 38 is grown to extend about 5 microns over the insulating oxide layer 30, as described in "Solid-Phase Lateral Epitaxial Growth onto Adjacent Si0 2 Film From Amorphous Silicon Deposited on Single-Crystal Silicon Substrate" by Y. Ohmura et al. , Jap. J. Appl. Physics Vol. 21, No. 3, March, 1982, pp. L152-L154; and "Lateral Solid-Phase Epitaxy of Silicon Over Oxide", J.A. Roth et al., MRS Symposium Proc. , Vol. 23 (1984), pp. 431-442. After the SPE seed (i.e., the SPE single crystal silicon film 38) has been formed (FIG. 6), a capping layer 42 of Si0 2 is deposited over the entire structure (FIG. 7). Then, with the SPE single 5 crystal silicon film 38 serving as a seed, seeded laser crystallization of the polysilicon film 40 using a laser beam 44 is carried out to extend the sinqle crystal silicon film 38 over the remaininq portion of the insulating layer 30. The laser power 0 which is used for the laser crystallization of the invention can be determined based only on the appropriate laser power for crystallizing polysilicon over an insulating oxide layer, without taking into account the laser power range for crystallizing 5 polysilicon on a silicon substrate. After laser crystallization has been completed to provide a single crystal silicon film 38 (FIG. 8) extendinq over the insulatinq layer 30, an Si0 2 /Si3 4 mask 46 is formed over the area on which : the final SOI device is to be formed and an oxide film 48 is grown over the seed area (FIG. 9). In the preferred embodiment, the oxide film 48 is a LOCOS oxide film. Finally, the Si0 2 /Si 3 N 4 mask 46 is stripped to leave isolated single crystal silicon 5 film structures 50 (corresponding to the single crystal silicon film 38) available for fabrication of the SOI device thereon (FIG. 10). As indicated above, the method of the present invention provides siqnificant advantaqes 0 over the prior art by employinq seeded laser crystallization after lateral solid phase epitaxial growth. That is, by providinq the SPE seed 38 (FIG. 6), the laser power window available for laser crystallization of the polvsilicon 40 over the 5 insulating layer 30 will have a much greater range than the laser power window available for prior art laser crystallization methods. In addition, the thickness of the insulating layer 30 on which the single crystal silicon film 38 is formed can be made greater than that available in prior art methods, so that a high quality SOI device, having improved isolation characteristics, can be formed on the sinqle crystal silicon film. The many features and advantaqes of the invention are apparent from the detailed specification and thus it is intended by the appended claims to cover all such features and advantages of the method which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modi ications and eαuivalents may be resorted to, fallinq within the scope of the invention.

Claims: WHAT IS CLAIMED IS: 1. A method of forming a sinqle crystal silicon film, comprising the steps of: (a) forming an insulating layer on a substrate, the insulating layer having a seed window formed therein to expose a portion of the substrate; (b) forming a layer of amorphous silicon on the insulating layer and on the portion of the substrate in the seed window; (c) heating the amorphous silicon layer so as to cause solid phase epitaxial growth of a solid phase epitaxial single crystal silicon film in the seed window and extending over a portion of the insulating layer, said heatinq step convertinq the remainder of the amorphous silicon layer to a polysilicon film; and (d) scanninq a laser beam across the polysilicon film to perform seeded laser crystallization usinq the solid phase epitaxial sinqle crystal silicon film as a seed, thereby extendinq the sinqle crystal silicon film over the remaining portion of the insulating layer. 2. A method as set forth in claim 1, wherein said step (c) comprises heating the amorphous silicon layer to a temperature of from 560 to 600°C. 3. A method as set forth in claim 2, wherein said step (c) further comprises heatinq the amorphous silicon layer by placing the substrate in an oven for isothermal heating to a temperature of substantially 560°C. 4. A method as set forth in claim 3, wherein said step (c) further comprises heating the amorphous silicon layer so that the solid phase epitaxial single crystal silicon film extends substantially 5 microns over the insulating layer. 5. A method as set forth in claim 2, wherein said step (c) further comprises heating the amorphous silicon layer so that the solid phase epitaxial single crystal silicon film extends substantially 5 microns over the insulating layer. 6. A method as set forth in claim 1, further comprising the step of depositing a capping layer on the polysilicon film and the solid phase epitaxial single crystal silicon film between said steps (c) and (d). 7. A method as set forth in claim 6, further comprising the steps of: (e) removinq the cappinq la er after said step (d); (f) forming a mask over the portion of the single crystal silicon film on which a device is to be formed; (q) forming an insulatinq film on the portions of the single crystal silicon film which are exposed by the mask; and (h) removinq the mask to expose the sinqle crystal silicon film on which the device is to be formed. 8. A method as set forth in claim 7, wherein said step (g) comprises growing a LOCOS oxide film. 9. A method as set forth in claim 8, wherein said step (f) comprises forming an Si0 2 /Si 3 N 4 mask over the portion of the single crystal silicon film on which a device will be formed. 10. A method as set forth in claim 7, wherein said step (c) comprises heating the amorphous silicon layer to a temperature of from 560 to 600 β C. 11. A method as set forth in claim 10, wherein said step (c) further comprises heating the amorphous silicon layer so that the solid phase epitaxial single crystal silicon film extends substantially 5 microns over the insulating layer. 12. A method as set forth in claim 7, wherein said step (c) comprises heating the amorphous silicon layer so that the solid-phase epitaxial single crystal silicon film extends substantially 5 microns over the insulating layer. 13. A method as set forth in claim 1, wherein said step (a) comprises growing a LOCOS oxide layer on the substrate. 14. A method as set forth in claim 13, wherein: said step (a) comprises qrowinq the insulating layer to be at least 1 micron thick; and said step (b) comprises forming the amorphous silicon layer to be substantially .2 to 5 microns thick. 15. A method of forming a single crystal silicon film, comprising the steps of: (a) forming an oxide insulating layer on a substrate, the oxide insulating layer having a seed window formed therein to expose a portion of the substrate; (b) forming a layer of amorphous silicon on the oxide insulating layer and on the portion of the substrate in the seed window; (c) heating the amorphous silicon layer so as to cause solid phase epitaxial growth of a solid phase epitaxial single crystal silicon film in the seed window and extending over a portion of the oxide insulating layer, said heating step converting the remainder of the amorphous silicon layer to a polysilicon film; (d) depositing a capping layer over the polysilicon film and the solid phase epitaxial single crystal silicon film; and (e) scanning a laser beam across the polysilicon film to perform seeded laser crystallization using the solid phase epitaxial single crystal silicon film as a seed, thereby extending the single crystal silicon film over the remaining portion of the oxide insulating layer. 16. A method as set forth in claim 15, wherein said step (c) comprises heating the amorphous silicon layer to a temperature of from 560 to 600 β C. 17. A method as set forth in claim 16, wherein said step (c) further comprises heating the amorphous silicon layer by placing the substrate in an oven for isothermal heating to a temperature of substantially 560 β C. 18. A method as set forth in claim 17, wherein said step (c) further comprises heating the amorphous silicon layer so that the solid phase epitaxial single crystal silicon film extends substantially 5 microns over the oxide insulating layer. 19. A method as set forth in claim 16, wherein said step (c) further comprises heating the amorphous silicon layer so that the solid phase epitaxial single crystal silicon film extends substantially 5 microns over the oxide insulatinq layer.

Inventors: CSERHATI ANDRUS FABRICIUS, DIEHL RICHARD BRUCE, GOETZ GEORGE GABRIEL, CSERHATI, ANDRUS, FABRICIUS, DIEHL, RICHARD, BRUCE, GOETZ, GEORGE, GABRIEL
Applicants: ALLIED CORP, ALLIED CORPORATION
Application year: 1986
Main classification: C30B13
IPC codes: C30B1, C30B13, C30B29, H01L21, H01L27