Abstract:
Provided is a method for manufacturing a semiconductor device. The method includes: providing a first substrate where an active layer is formed on a buried insulation layer; forming a gate insulation layer on the active layer; forming a gate electrode on the gate insulation layer; forming a source/drain region on the active layer at both sides of the gate electrode; exposing the buried insulation layer around a thin film transistor (TFT) including the gate electrode and the source/drain region; forming an under cut at the bottom of the TFT by partially removing the buried insulation layer; and transferring the TFT on a second substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application No. 10-2009-0120620, filed on Dec. 7, 2009, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention disclosed herein relates to a method for manufacturing a semiconductor device, and more particularly, to a method for manufacturing a semiconductor device, which forms a thin film transistor (TFT) on a plastic substrate. 
     In general, an Organic Thin Film Transistor (OTFT) is extensively used in a flexible display driving device or a Radio Frequency Identification (RFID) application device. When an organic material is used for a channel layer in the OTFT, since conduction mechanism and crystallity are defective, mobility of more than 1 cm 2 /Vs may not be easily realized. Although the OTFT is used for realizing a flexible electronic device, since it has a short life cycle and its driving reliability is deteriorated while being exposed to atmosphere, it is difficult to achieve mass production. 
     Accordingly, due to a technical deadlock state of the OTFT with limitations related to a life cycle and reliability and its increased demand for a special purpose high-speed flexible device, recently suggested is an alternative technique that an existing silicon substrate semiconductor is detached from a wafer substrate and is transferred on a plastic substrate. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for manufacturing a semiconductor, which can increase or maximize production yield by completing the formation of a thin film transistor (TFT) on a semiconductor substrate and then transferring the TFT on a plastic substrate. 
     Embodiments of the present invention provide methods for manufacturing a semiconductor device, the methods including: providing a first substrate where an active layer is formed on a buried insulation layer; forming a gate insulation layer on the active layer; forming a gate electrode on the gate insulation layer; forming a source/drain region on the active layer at both sides of the gate electrode; exposing the buried insulation layer around a thin film transistor (TFT) including the gate electrode and the source/drain region; forming an under cut at the bottom of the TFT by partially removing the buried insulation layer; and transferring the TFT on a second substrate. 
     In some embodiments, the gate insulation layer and the buried insulation layer may be formed of insulation layer materials of respectively different kinds. 
     In other embodiments, the buried insulation layer may be formed of a silicon oxide layer and the gate insulation layer is formed of a silicon nitride layer. 
     In still other embodiments, the exposing of the buried insulation layer may include: forming a photoresist pattern on the TFT; and etching the gate insulation layer by using the photoresist pattern as an etching mask. 
     In even other embodiments, the gate insulation layer may be etched using a dry etching method. 
     In yet other embodiments, the dry etching method may use CF-based gas. 
     In further embodiments, the buried insulation layer may be removed using a wet etching method that uses the photoresist pattern and the gate insulation layer as an etching mask. 
     In still further embodiments, the wet etching method of the buried insulation layer may use buffered HF solution. 
     In even further embodiments, after the etching of the buried insulation layer, the methods may further include performing a hard bake process on the photoresist pattern. 
     In yet further embodiments, the methods may further include fixing the photoresist pattern at a stamp. 
     In yet further embodiments, the stamp may include Polydimethylsiloxane (PDMS) on a surface that contacts the photoresist pattern. 
     In yet further embodiments, the photoresist pattern may be exposed to UltraViolet (UV) before being fixed at the stamp. 
     In yet further embodiments, the photoresist pattern may be removed by development solution after the TFT is transferred on the second substrate. 
     In yet further embodiments, the gate insulation layer and the gate electrode may be formed into a gate stack on the active layer. 
     In yet further embodiments, after the forming of the source/drain region, the methods may further include forming an interlayer insulation layer that is formed of a material different from the buried insulation layer on the gate electrode and the active region. 
     In yet further embodiments, the buried insulation layer may be formed of a silicon oxide layer and the interlayer insulation layer may be formed of a silicon nitride layer. 
     In yet further embodiments, the exposing of the buried insulation layer may include: forming a photoresist pattern on the interlayer insulation layer above the TFT; and etching the interlayer insulation layer by using the photoresist pattern as an etching mask. 
     In yet further embodiments, the interlayer insulation layer may be etched using a dry etching method. 
     In yet further embodiments, the buried insulation layer may be removed by a wet etching method that uses the photoresist pattern and the interlayer insulation layer as an etching mask. 
     In yet further embodiments, the methods may further include separating the active layer before the forming of the gate insulation layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present invention and, together with the description, serve to explain principles of the present invention. In the drawings: 
         FIGS. 1 through 11  are manufacturing sectional views illustrating a method for manufacturing a semiconductor device according to a first embodiment of the present invention; and 
         FIGS. 12 through 23  are manufacturing sectional views illustrating a method for manufacturing a semiconductor device according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. 
     Preferred embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. 
     In the drawings, the dimensions of layers and regions are exaggerated for clarity of illustration. It will also be understood that when a layer (or film) is referred to as being ‘on’ another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being ‘under’ another layer, it can be directly under, and one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being ‘between’ two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. Like reference numerals refer to like elements throughout. 
     Hereinafter, first and second embodiments will be described about an exemplary embodiment of the present invention in conjunction with the accompanying drawings. 
     First Embodiment 
     In relation to a method for manufacturing a semiconductor device according to a first embodiment of the present invention, since a buried insulation layer of a Silicon On Insulator (SOI) substrate and a gate insulation layer of a Thin Film Transistor (TFT) are formed of respectively different kinds of materials, the method prevents the gate insulation layer from being damaged by an etching solution of the buried insulation layer during separating of the TFT from the SOI substrate. 
       FIGS. 1 through 11  are manufacturing sectional views illustrating the method for manufacturing a semiconductor device according to the first embodiment of the present invention. 
     Referring to  FIG. 1 , a first substrate  10 , where a buried insulation layer  14  and an active layer  16  are stacked, is prepared on a single crystalline silicon wafer  12 . The buried insulation layer  14  and the active layer  16  may be formed on the single crystalline silicon wafer  12 . The buried insulation layer  14  includes a silicon oxide layer and the active layer  16  includes a single crystalline silicon layer. For example, the first substrate  10  may include a SOI substrate with the active layer  16  whose thickness is about 290 nm. 
     Referring to  FIG. 2 , the active layer  16  of the first substrate  10  is divided. Here, the active layer  16  may be formed of an opaque single crystalline silicon thin layer used as a channel layer of a TFT. Accordingly, an island process of the active layer  16  may be a process that is required for improving transmittivity in a display device using a TFT. For example, the active layer  16  may be divided through a first photolithography process and a first etching process. 
     Referring to  FIG. 3 , a gate insulation layer  18  is formed on an entire surface of the first substrate  10  including the active layer  16 . The gate insulation layer  18  may include a silicon nitride (SiN) layer formed using a Chemical Vapor Deposition (CVD) method or an Atomic Layer Deposition (ALD) method. The CVD method or the ALD method is performed at a high temperature of more than about 200° C. For example, the gate insulation layer  18  may be formed with a thickness of about 30 Å to about 300 Å. 
     Referring to  FIG. 4 , a gate electrode  20  is formed on the gate insulation layer  18  above the active layer  16 . The gate electrode  20  may include conductive metal, poly silicon doped with conductive impurity, or metal silicide, which is formed on the gate insulation layer  18 . The gate electrode  20  is a place to which a switching voltage for inducing a channel in the active layer  16  below the gate insulation layer  18  is applied and may be separately patterned above the active layer  16 . For example, after a conductive metal is deposited on the gate insulation layer  18 , the gate electrode  20  is formed through a second photolithography process and a second etching process. 
     Referring to  FIG. 5 , source/drain impurity regions  22  and  24  are formed by ion-implanting conductive impurity on active regions at both sides of the gate electrode  20 . The source/drain impurity regions  22  and  24  may be formed through a self align ion implantation method that uses the gate electrode  20  as an ion implantation mask. During the ion implantation, the conductive impurity may be ion-implanted into the active layer  16  through the gate insulation layer  18 . 
     In addition, the conductive impurity may include a p-type impurity of group III elements such as B, Ga, and In, and an n-type impurity of group V elements such as Sb, As, and P. The TFT  21  may be formed into a P-type Metal Oxide Semiconductor (PMOS) transistor or an N-type Metal Oxide Semiconductor (NMOS) transistor, depending on kinds of conductive impurities that are ion-implanted into the source/drain impurity regions  22  and  24 . When PMOS and NMOS TFTs  21  are simultaneously designed on the first substrate  10 , it is necessary to sequentially ion-implant conductive impurities of different kinds into respectively different regions. Each time conductive impurity is ion-implanted, a photolithography process is necessarily required in order to form an ion implantation mask that selectively exposes a corresponding region. Accordingly, when the PMOS and NMOS TFTs  21  are realized on the first substrate  10  at the same time, a plurality of photolithography processes and ion implantation processes are additionally required. 
     Referring to  FIG. 6 , a photo resist pattern  26  is formed on TFT  21  including the gate electrode  20  and the source/drain impurity regions  22  and  24 . Here, the photoresist pattern  26  may be formed on the TFT  21  and the active layer  16  through a third photolithography process. 
     Referring to  FIG. 7 , the buried insulation layer  14  of the first substrate  10  is exposed by removing the gate insulation layer  18  that is exposed by the photoresist pattern  26 . Here, the gate insulation layer  18  may be removed through a third etching process that uses the photoresist pattern  26  as an etching mask. The third etching process may be performed through a dry etching method that removes the gate insulation layer  18  anisotropically. At this point, the gate insulation layer  18  may be removed by an etching gas that has a high etch selectivity with respect to the buried insulation layer  14 . For example, the etching gas of the silicon nitride layer may include CF-based gas such as CF4, CH2F2, and CHF3. 
     Referring to  FIG. 8 , an under cut is formed at the bottom of the TFT  21  by removing the buried insulation layer  14  isotropically. At this point, a portion of the buried insulation layer  14  remains to support the TFT  21  on the wafer  12 . The buried insulation layer  14  may be removed by a fourth etching process that uses the photoresist pattern  26  and the gate insulation layer  18  as an etching mask. For example, the fourth etching process may be performed through a wet etching method that removes buried insulation layer  14  isotropically. For example, the buried insulation layer  14  may be isotropically removed by an etching solution with buffered HF. The buffered HF solution is provided through which the buried insulation layer  14  of the silicon oxide layer has a higher etch selectivity than the gate insulation layer  18  of the silicon nitride layer. 
     Accordingly, the method of manufacturing a semiconductor device according to the first embodiment of the present invention uses a wet etching method that uses an etching solution through which the buried insulation layer  14  has a higher etch selectivity than the gate insulation layer  4 , and thus does not damage the gate insulation layer  18  and allows the TFT  21  to be separated from the first substrate  10  without difficulties. 
     Next, a hard bake process may be further performed on the photoresist pattern  26  that is used as an etching mask during the fourth etching process. The reason is that the photoresist pattern  26  may be damaged or softened due to moisture during the third and fourth etching processes such that stamp adhesion of a next process may become difficult. 
     Referring to  FIG. 9 , the photoresist pattern  26  on the TFT  21  is fixed at the stamp  28 , and the TFT  21  is separated from the first substrate  10 . The stamp  28  is attached to the photoresist pattern  26  to physically fix the TFT  21  at the bottom of the photoresist pattern  26 , and to separate the TFT  21  from the first substrate  10 . For example, the stamp  28  may include an adhesive material having excellent adhesiveness such as Polydimethylsiloxane (PDMS) at the surface contacting the photoresist pattern  26 . Additionally, the stamp  28  includes a substrate such as glass and an adhesive including PDMS at the surface of the substrate. 
     Referring to  FIG. 10 , the TFT  21  is transferred on a second substrate  30  on which an adhesive is coated. Here, the second substrate  30  may include a transparent and flexible plastic substrate. Additionally, the adhesive includes a petrochemical adhesive such as epoxy, silicon, het melt, PVAc, etc. 
     Referring to  FIG. 11 , the stamp  28  and the photoresist pattern  26  are removed. The photoresist pattern  26  may be easily removed by a volatile solvent such as alcohol. In addition, an adhesive between the second substrate  30  and the TFT  21  can be also melted by a volatile solvent. The photoresist pattern  26  is exposed to UltraViolet (UV) first before being fixed to the stamp  28 , and may be removed by development solution, which is different from an adhesive. 
     Although not shown, a first interlayer insulation layer is deposited on the TFT  21  and a contact hole is formed to expose the source/drain impurity regions  22  and  24  through a fourth photolithography process and a fifth etching process. Moreover, a conductive metal layer is deposited in the contact hole and on the interlayer insulation layer, and wirings including source/drain electrodes are formed through a fifth photolithography process and a sixth etching process. Furthermore, a second interlayer insulation layer and wirings may be further formed on the second substrate  30 . 
     As a result, the method for manufacturing a semiconductor device according to the first embodiment of the present invention prevents a damage of the gate insulation layer  18  when the TFT  21  is separated from the first substrate  10  since the gate insulation layer  18  is formed of a material different from the buried insulation layer  14  of the first substrate  10 . 
     Additionally, after the TFT  21  that requires a manufacturing process of a high temperature is formed on the first substrate  10 , it can be transferred on the second substrate  30  that is relatively vulnerable to a high temperature. Thus, production yield can be improved. 
     Second Embodiment 
     In relation to a method for manufacturing a semiconductor device according to a second embodiment of the present invention, a gate electrode and a gate insulation layer are formed into a gate stack and an interlayer insulation layer is formed on the gate stack. Thus, the method prevents the gate insulation layer from being damaged by an etching solution of a buried insulation layer during separating of a TFT from a SOI substrate. 
       FIGS. 12 through 23  are manufacturing sectional views illustrating the method for manufacturing a semiconductor device according to the second embodiment of the present invention. 
     Referring to  FIG. 12 , a first substrate  10 , where a buried insulation layer  14  and an active layer  16  are stacked, is prepared on a wafer  12 . The buried insulation layer  14  includes a silicon oxide layer and the active layer  16  includes a single crystalline silicon layer. For example, the first substrate  10  may include a SOI substrate with the active layer  16  whose thickness is about 290 nm. 
     Referring to  FIG. 13 , the active layer  16  of the first substrate  10  is divided. Here, the active layer  16  may be formed of an opaque single crystalline silicon thin layer used as a channel layer of a TFT. Accordingly, an island process of the active layer  16  may be a process required for improving transmittance in a display device using TFT. For example, the active layer  16  may be divided by a first photolithography process and a first etching process. 
     Referring to  FIG. 14 , a gate insulation layer  18  and a gate electrode  20  are formed on an entire surface of the first substrate  10  including the active layer  16 . The gate insulation layer  18  includes a silicon oxide layer. The gate insulation layer  18  may be formed through a rapid thermal process method requiring an atmosphere of a high temperature of more than about 200° C. or a CVD method. The gate electrode  20  may include silicon doped with conductive impurity, conductive metal, or metal silicide, which is formed through a CVD method or a sputtering method. 
     Referring to  FIG. 15 , a gate stack including the gate insulation layer  18  and the gate electrode  20  is formed on the active layer  16 . Here, the gate electrode  20  and the gate insulation layer  18  may be patterned into the gate stack  19  on the active layer  16  through a second photolithography process and a second etching process. The second etching process may be performed through a dry etching method that removes the gate electrode  20  and the gate insulation layer  18  anisotropically. 
     Referring to  FIG. 16 , source/drain impurity regions  22  and  24  are formed by ion-implanting conductive impurity on active regions  16  at both sides of the gate stack  19 . The source/drain impurity regions  22  and  24  may be formed through a self align ion implantation method that uses the gate stack  19  as an ion implantation mask. As mentioned above, the conductive impurity may include a p-type impurity and an n-type impurity. The TFT  21  may be formed into a PMOS transistor or an NMOS transistor, depending on kinds of conductive impurities that are ion-implanted on the source/drain impurity regions  22  and  24 . When PMOS and NMOS TFTs  21  are simultaneously designed on the first substrate  10 , it is necessary to sequentially ion-implant conductive impurities of different kinds on respectively different regions. Each time conductive impurity is ion-implanted, a photolithography process is necessarily required to form an ion implantation mask that selectively exposes a corresponding region. Accordingly, when the PMOS and NMOS TFTs  21  are realized on the first substrate  10  at the same time, a plurality of photolithography processes and ion implantation processes are additionally required. 
     Refer to  FIG. 17 , an interlayer insulation layer  40  is formed on an entire surface of the first substrate  10 . Here, the interlayer insulation layer  40  may include a silicon nitride layer. The interlayer insulation layer  40  may be formed using a CVD method of a high temperature. In addition, the interlayer insulation layer  40  may cover the gate insulation layer  18  exposed at the gate stack  19  on the active layer  16 . 
     Referring to  FIG. 18 , a photoresist pattern  26  is formed on the interlayer insulation layer  40  above the TFT  21  including the gate electrode  20  and the source/drain impurity regions  22  and  24 . Here, the photoresist pattern  26  may be formed above the TFT  21  and the active layer  16  through a third photolithography process. 
     Referring to  FIG. 19 , the buried insulation layer  14  of the first substrate  10  is exposed by anisotropically removing the interlayer insulation layer  40  that is exposed by the photoresist pattern  26 . Here, the gate insulation layer  18  may be removed through a third etching process that uses the photoresist pattern  26  as an etching mask. The third etching process may be performed through a dry etching method. At this point, the interlayer insulation layer  40  may be removed by an etching gas that has a high etch selectivity with respect to the buried insulation layer  14 . For example, the etching gas of the silicon nitride layer may include CF-based gas such as CF4, CH2F2, and CHF3. 
     Referring to  FIG. 20 , an under cut is formed by removing the buried insulation layer  14  isotropically. At this point, a portion of the buried insulation layer  3  remains to support the TFT  21  on the wafer  12 . The buried insulation layer  14  may be removed by a fourth etching process that uses the photoresist pattern  26  and the interlayer insulation layer  40  as an etching mask. For example, the fourth etching process may be performed through a dry etching method. For example, the buried insulation layer  14  may be isotropically removed by an etching solution with buffered HF. The buffered HF solution is provided through which the buried insulation layer  14  of the silicon oxide layer has a higher etch selectivity than the interlayer insulation layer  40  of the silicon nitride layer. At this point, the gate insulation layer  18  formed of the same silicon oxide layer as the buried insulation layer  14  may be protected from etching solution by the interlayer insulation layer  40 . 
     Accordingly, the method of manufacturing a semiconductor device according to the second embodiment of the present invention protects the gate insulation layer  18  from the etching solution of the buried insulation layer  14  when the TFT  21  is separated because the gate insulation layer  18  and the gate electrode  20  are formed into the gate stack  19  and the interlayer insulation layer  40  is formed on the gate stack  19 . 
     Next, a hard bake process may be further performed on the photoresist pattern  26  that is used as an etching mask during the fourth etching process. The reason is that the photoresist pattern  26  may be damaged or softened due to moisture through the third and fourth etching processes such that stamp adhesion of a next process may become difficult. 
     Referring to  FIG. 21 , the photoresist pattern  26  on the TFT  21  is fixed at the stamp  28 , and the TFT  21  is separated from the first substrate  10 . The stamp  28  is attached to the photoresist pattern  26  to physically fix the TFT  21  at the bottom of the photoresist pattern  26 , and to separate the TFT  21  from the first substrate  10 . For example, the stamp  28  may include an adhesive material having excellent adhesiveness such as PDMS at the surface contacting the photoresist pattern  26 . Additionally, the stamp  28  includes a substrate such as glass and an adhesive including PDMS at the surface of the substrate. 
     Referring to  FIG. 22 , the TFT  21  is transferred on a second substrate  30  on which an adhesive is coated. Here, the second substrate  30  may include a transparent and flexible plastic substrate. Additionally, the adhesive includes a petrochemical adhesive such as epoxy, silicon, het melt, PVAc, etc. 
     Referring to  FIG. 23 , the stamp  28  and the photoresist pattern  26  are removed. The photoresist pattern  26  may be easily removed by a volatile solvent such as alcohol. In addition, an adhesive between the second substrate  30  and the TFT  21  can be also melted by a volatile solvent. After the photoresist pattern  26  is exposed to UV first before being fixed to the stamp  28 , it may be removed by development solution, which is different from an adhesive. 
     Although not shown, a second interlayer insulation layer is deposited on the TFT  21  and a contact hole is formed to expose the source/drain impurity regions  22  and  24  through a fourth photolithography process and a fifth etching process. Moreover, a conductive metal layer is deposited in the contact hole and on the interlayer insulation layer, and wirings including a source/drain electrode are formed through a fifth photolithography process and a sixth etching process. Furthermore, a third interlayer insulation layer and wirings may be further formed on the second substrate  30 . 
     As a result, the method of manufacturing a semiconductor device according to the second embodiment of the present invention prevents the damage of the gate insulation layer  18  during separating of the TFT  21  from the first substrate  10  since the interlayer insulation layer covers the gate insulation layer  18  even if the buried insulation layer  14  and the gate insulation layer  18  are formed of the same material. 
     Additionally, after the TFT  21  that requires a manufacturing process of a high temperature is formed on the first substrate  10 , since it can be transferred on the second substrate  30  that is relatively vulnerable to a high temperature. Thus, production yield can be improved. 
     According to the embodiments of the present invention, since a buried insulation layer and a gate insulation layer of a SOI substrate are formed of respectively different kinds of materials, damage of the gate insulation layer can be prevented when the buried insulation layer is removed during separating of a TFT. Therefore, production yield can be increased. 
     Additionally, an interlayer insulation layer is formed on a gate stack including a gate electrode and a gate insulation layer so such that the gate insulation layer is not exposed to an etching solution of the buried insulation layer during separating of a TFT. Therefore, production yield can be maximized. 
     The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the true spirit and scope of the present invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.