Method of producing a wafer with an epitaxial quality layer and device with epitaxial quality layer

[0002] (1) Field of the Invention

[0003] The present invention is related to semiconductor fabrication. More specifically the present invention is related to fabrication of epitaxial (EPI) quality wafers.

[0004] (2) Background Information

[0005] Epitaxial (EPI) quality wafers (hereinafter referred to as “EPI wafers”) are well-known in the art. The term “epitaxial” is defined as the growth of a single-crystal semiconductor film upon a single-crystal substrate. An epitaxial layer has the same crystallographic characteristics as the substrate material. The single-crystalline epitaxial structure comes about when silicon atoms are deposited on a bare silicon wafer in a Chemical Vapor Deposition (“CVD”) reactor. When chemical reactants are controlled and the system parameters are set correctly, the depositing atoms arrive at the wafer surface with sufficient energy to move around on the surface and orient themselves to the crystal arrangement of the wafer atoms.

[0006] EPI wafers typically include a substrate of a heavily doped silicon onto which a layer of approximately 1 to 3 microns is epitaxially grown. FIG. 1 illustrates a cross sectional view through an EPI wafer onto which a transistor 105 (shown within dotted lines) is built. Transistor 105 has a gate 106 formed on EPI layer 104 and active regions 108 and 110 formed into EPI layer 104. The EPI wafer includes a heavily doped silicon substrate 102 on which an epitaxial low doped layer 104 of 1-3 micrometers is grown. EPI layer 104 by definition has a low doping which makes it very desirable for building transistors. Layer 104's low doping leads to substantial reduction in the transistor's junction capacitances C₁and C₂. Typically, the smaller the junction capacitances of a transistor, the faster the transistor is. Because junction capacitances are proportional to the doping of layer 104, junction capacitances C₁and C₂are relatively low.

[0007] While EPI layers present the advantage explained above in connection with their lower doping, EPI wafers are very costly. Conventional EPI wafers may run up to approximately $200.00. Moreover, while several methods have been proposed to obtain EPI wafers at a cheaper cost, the proposed methods are limited to providing transistors out of EPI layers that have thicknesses smaller than 1 micrometer. EPI layers having thicknesses of less than 1 micrometer cause the transistors fabricated onto these layers to be limited in performance as these transistors have relatively large junction capacitances. The large junction capacitances are due, mainly, to the fact that as the EPI layer is thin, the junction capacitances extend in the heavily doped substrate.

[0008] An embodiment of the present invention includes a method of making a wafer. A first semiconductor film is formed onto a semiconductor substrate. An epitaxial film is formed on an epitaxial wafer. The epitaxial wafer is placed with the epitaxial film on the first semiconductor film. The epitaxial film is debonded from the epitaxial wafer. The epitaxial film is bonded to the first semiconductor film.

[0009] The features, aspects, and advantages of the present invention will become more fully apparent from the following detailed Description, appended claims, and accompanying drawings in which:

[0010] FIG. 1 illustrates in block diagram form, a cross sectional view through an epitaxial-quality (EPI) wafer onto which a transistor is built;

[0011] FIG. 2 illustrates a cross sectional view through a semiconductor substrate utilized in the embodiment of the method of fabricating a wafer according to the present invention;

[0012] FIG. 3 illustrates a cross sectional view showing a film deposited over the semiconductor substrate according to one embodiment of the method of fabricating a wafer of the present invention;

[0013] FIG. 4 illustrates a cross-sectional view through an epitaxial (EPI) wafer subjected to a process of ion implantation, utilized in the embodiment of the method of fabricating a wafer according to the present invention;

[0014] FIG. 5 illustrates a cross-sectional view through a structure that includes the EPI wafer with an embrittled EPI film placed with the embrittled EPI film onto the semiconductor film of the first semiconductor substrate;

[0015] FIG. 6 illustrates the remaining EPI wafer and the embrittled EPI film bonded to the first semiconductor film;

[0016] FIG. 7 illustrates the first semiconductor substrate with the first semiconductor film and the epitaxial film bonded to the first semiconductor film with a transistor formed in the epitaxial film; and

[0017] FIG. 8 illustrates a flow chart diagram in connection with the embodiment of a method of fabricating a wafer according to the present invention.

[0018] In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, one having ordinary skill in the art should recognize that the invention may be practiced without these specific details. In some instances, well-known circuits, structures, and techniques have not been shown in detail to avoid obscuring the present invention.

[0019] One embodiment of the present invention includes a method of producing a wafer with an epitaxial-quality layer. A first semiconductor film is formed onto a heavily doped semiconductor substrate. An epitaxial (EPI) film is formed onto an EPI wafer. The EPI wafer is placed with the EPI film on the first semiconductor film. The EPI film is debonded from the EPI wafer. The EPI film is bonded to the first semiconductor film. A transistor is formed onto the EPI film. The junction capacitances of this transistor are not formed in the heavily doped substrate, but rather in the EPI film above the first semiconductor film. Due to its lack of doping or light doping, the first semiconductor film does not cause the junction capacitances to be large.

[0020] FIGS. 2-7 illustrate cross-sectional views through several structures related to the fabrication of a wafer according to one embodiment of a method of the present invention. FIG. 2 illustrates a heavily doped semiconductor substrate 102. In one embodiment, the concentration of the doping of semiconductor substrate 102 is typically larger than 10¹⁸/cm³. Also, in one embodiment according to the present invention, the heavily doped semiconductor substrate 102 is a P+ heavily doped silicon substrate, but the present invention is not limited in scope to the use of a P+ heavily doped silicon substrate.

[0021] A semiconductor film 104 is deposited over the heavily doped semiconductor substrate 102 as illustrated in FIG. 3. Semiconductor film 104 may be not doped at all or just lightly doped such that when doped lightly its doping is substantially lower than a doping of the heavily doped substrate 107. In one embodiment, the doping of film 104 is lower than 10¹⁵/cm³. Also, in one embodiment of the present invention, semiconductor film 104 includes polysilicon. The deposition of polysilicon film 104 may be performed by way of a chemical vapor deposition (CVD) process which is well-known in the art. Polysilicon film 104 may have a thickness of approximately 2 micrometers. However, depending on the desired thickness of the film that will be finally formed on top of the heavily doped semiconductor substrate 102, the thickness of polysilicon film 104 may be ranging from a little over 0 to approximately 2 micrometers, although 2 micrometers is not limiting. Layer 104 may be alternatively made of a polysilicon amorphous semiconductor material.

[0022] FIG. 4 illustrates a cross-sectional view through an (EPI) wafer 106. The EPI wafer 106 is subjected to a process of hydrogen implantation. Hydrogen ions, having a concentration in a range of approximately 5×10¹⁶-10¹⁷/cm², are used to implant (bombard) the EPI wafer 106. The implantation of hydrogen ions causes a damaged surface 108 in the EPI wafer 106 close to top surface 117 of EPI wafer 106. The damaged surface 108 and top surface 117 of EPI wafer 106 defines an embrittled EPI film 110 that may later be bonded to the structure having the cross-section illustrated in FIG. 3. The embrittled EPI film 110 has a thickness of approximately 1 micrometer, but the present invention is not limited in scope to this value for the thickness of EPI film 108. A combined thickness of the first film and of the EPI film has a size such that a junction capacitance, of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) formed onto the EPI film, extends no further than the EPI film and the first film.

[0023] The EPI wafer 106 with embrittled film 110 is then turned upside down and placed with embrittled semiconductor film 110 onto the first semiconductor film 104 of wafer 102 as shown in FIG. 5. The assembly of FIG. 5 is then heated to a temperature that causes debonding of the remaining part 107 of EPI wafer 106 from the embrittled EPI film 110 as shown in FIG. 6. In one embodiment of the method of the present invention the temperature utilized for causing the above-mentioned debonding is approximately 400° C. The structure of FIG. 6 is further heated to a temperature that causes bonding of epitaxial film 110 to first semiconductor film 104. Such temperature may be approximately 800° C.

[0024] The structure 111, including epitaxial film 110 bonded to first film 104, of the semiconductor substrate 102, is then planarized by way of a well-known Chemical Mechanical Polishing (CMP) process. The net result is a heavily doped substrate P+ 102 with a layer 142 including the two films 110 and 104. After the CMP process, an active device such as a transistor 130 may be built onto the structure 111 as illustrated in FIG. 7. Transistor 130 (shown within dotted lines) has a gate 124 formed on top of film 110 and active region 126 and 128 formed in the epitaxial film 110. The amorphous silicon layer or polysilicon layer 104 is typically not doped such that the junction capacitances of transistor 130 are relatively low.

[0025] Layer 142 has a thickness greater than 1 micrometer. This causes the junction capacitance of transistor 130 to be formed mostly in the epitaxial film 110 and in the undoped amorphous silicon or polysilicon film 104. The thickness of layer 142 which is in the range of approximately 1-3 μm provides a medium thick enough to prevent the formation of junction capacitances in a medium that includes heavily doped substrate 102.

[0026] FIG. 8 illustrates a flow chart diagram in connection with the embodiment of a method of fabricating a wafer according to the present invention. The method starts at step 802 from where it passes to block 804. At block 804 a first semiconductor film is formed onto a semiconductor substrate. The first semiconductor film is generally not doped. The method then passes to block 806 where an epitaxial film is formed onto an epitaxial wafer. The method then passes to block 808 where the epitaxial wafer is placed with an epitaxial film onto the first semiconductor film. Then next, at block 810 the epitaxial film is debonded from the epitaxial wafer. Next, at block 810, the epitaxial film is bonded to the first semiconductor film.

[0027] In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will however be evident that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Therefore, the scope of the invention should be limited by the appended claims.