Title of Invention: Semiconductor device with transistor, conductive pad, and contact Abstract: A semiconductor device includes a transistor, a conductive pad, and a contact. The conductive pad is electrically connected to the transistor. The conductive pad may include, but is not limited to, a first region and a second region. The contact is electrically connected to the conductive pad. At least a main part of the first region overlaps the transistor in plan view. At least a main part of the second region does not overlap the transistor in plan view. At least a main part of the contact overlaps the second region in plan view. The at least main part of the contact does not overlap the first region in plan view. The at least main part of the contact does not overlap the transistor in plan view. Detailed Description: Part general_description_paragraphs: Part BACKGROUND OF THE INVENTION: 1. Field of the Invention The present invention generally relates to a semiconductor device and a method of forming the same. More specifically, the present invention relates to a semiconductor device including a vertical transistor and a method of forming the same. Priority is claimed on Japanese Patent Application No. 2008-295831, filed Nov. 19, 2008, the content of which is incorporated herein by reference. 2. Description of the Related Art There have been known vertical metal oxide semiconductor transistors which will hereinafter be referred to as vertical MOS transistor. Japanese Unexamined Patent Application, First Publication, No. 2005-012214 discloses a vertical nMOSFET which includes an n+-relaxed SiGe drain, an epitaxial p-relaxed SiGe body, an epitaxial n+-tensile stressed Si, and an epitaxial tensile stressed channel. Japanese Unexamined Patent Application, First Publication, No. 11-214684 discloses a semiconductor device including a MOS transistor. The MOS transistor includes an n-diffusion wiring layer. The MOS transistor also includes an n+-silicon selective epitaxial layer as a source region over the n-diffusion wiring layer. The MOS transistor also includes a p−-silicon selective epitaxial layer as a channel region over the n-diffusion wiring layer. The MOS transistor also includes an n+-silicon selective epitaxial layer as a drain region over the n-diffusion wiring layer. Part SUMMARY: In one embodiment, a semiconductor device may include, but is not limited to, a transistor, a conductive pad, and a contact. The conductive pad is electrically connected to the transistor. The conductive pad may include, but is not limited to, a first region and a second region. The contact is electrically connected to the conductive pad. At least a main part of the first region overlaps the transistor in plan view. At least a main part of the second region does not overlap the transistor in plan view. At least a main part of the contact overlaps the second region in plan view. The at least main part of the contact does not overlap the first region in plan view. The at least main part of the contact does not overlap the transistor in plan view. In another embodiment, a method of forming a semiconductor device may include, but is not limited to, forming a transistor, forming a conductive pad, reducing a resistance of the conductive pad, and forming a contact. The conductive pad may include, but is not limited to, a first region and a second region. The conductive pad is electrically connected to the transistor. At least a main part of the first region overlaps the transistor in plan view. At least a main part of the second region not overlaps the transistor in plan view. At least a main part of the contact overlaps the second region in plan view. The at least main part of the contact does not overlap the first region in plan view. The at least main part of the contact does not overlap the first region in plan view. In still another embodiment, a method of forming a semiconductor device may include, but is not limited to, forming a transistor, forming a conductive pad, forming a mask with a contact hole, introducing a dopant, diffusing the dopant, and forming a contact. The conductive pad may include, but is not limited to, a first region and a second region. The conductive pad is electrically connected to the transistor. At least a main part of the first region overlaps the transistor in plan view. At least a main part of the second region does not overlap the transistor in plan view. A mask with a contact hole is formed. The mask covers the conductive pad. A dopant through the contact hole is introduced into a selected part of the conductive pad. At least a main part of the selected part does not overlap the transistor in plan view. The dopant is diffused from the selected part to the conductive pad entirely. A contact is formed. At least a main part of the contact overlaps the second region in plan view. The at least main part of the contact does not overlap the first region in plan view. The at least main part of the contact does not overlap the first region in plan view. Part DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: Before describing the present invention, the related art will be explained in detail with reference to FIGS. 46A, 46B, 47A, 47B, 48A, 48B, 49A, and 49B, in order to facilitate the understanding of the present invention. FIG. 49A is a fragmentary cross sectional elevation view, taken along a first direction, illustrating a semiconductor device in the related art. FIG. 49B is a fragmentary cross sectional elevation view, taken along a second direction perpendicular to the first direction, illustrating the semiconductor device of FIG. 49A. FIG. 46A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the related art in a step involved in a method of forming the semiconductor device of FIG. 49A. FIG. 46B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 46A. FIG. 47A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the related art in a step subsequent to the step of FIG. 46A involved in the method of forming the semiconductor device of FIG. 49A. FIG. 47B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 47A. FIG. 48A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the related art in a step subsequent to the step of FIG. 47A involved in the method of forming the semiconductor device of FIG. 49A. FIG. 48B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 48A. With reference to FIGS. 49A and 49B, a semiconductor device includes a semiconductor substrate 1, lower diffusion layers 6, pillars 2, upper diffusion layers 13, gate insulating films 8, and gate electrodes 7. The lower diffusion layers 6 are disposed in the semiconductor substrate 1. The pillars 2 are disposed over the lower diffusion layers 6. The pillars 2 provide channels. The upper diffusion layers 13 are disposed over the pillars 2. The gate insulating film 8 surrounds the pillar 2. The gate electrode 7 surrounds the gate insulating film 8 that surrounds the pillar 2. The gate electrode 7 is separated by the gate insulating film 8 from the pillar 2. The lower diffusion layers 6 are electrically separated by oxide films 5 and 5b from the gate electrodes 7. The upper diffusion layer 13 is surrounded by a side wall 9. The upper diffusion layer 13 is electrically separated by an oxide film 9a from the gate electrodes 7. Contact layers 12 are disposed over the upper diffusion layers 13. The contact layers 12 are doped with a dopant. An inter-layer insulator 11 extends over the contact layers 12 and over the gate electrodes 7. The inter-layer insulator 11 has contact holes in which contacts 14 are disposed. The contacts 14 are disposed over the contact layers 12. The upper diffusion layer 13 is electrically connected through the contact layer 12 to the contact 14. The contact layer 12 doped with the dopant is provided to reduce the contact resistance between the upper diffusion layer 13 and the contact 14. Processes for forming the semiconductor device of FIGS. 49A and 49B will be described. With reference to FIGS. 46A and 46B, a vertical transistor structure is formed over the semiconductor substrate 1. The vertical transistor structure includes the lower diffusion layers 6, the pillars 2, the gate insulating films 8, the gate electrodes 7, and semiconductor layers 24. The semiconductor layers 24 are formed into the contact layers 12 and the upper diffusion layers 13. The inter-layer insulator 11 is formed over the vertical transistor structure. With reference to FIGS. 47A and 47B, a resist pattern is formed over the inter-layer insulator 11 by a lithography process. A dry etching process is carried out using the resist pattern as a mask to form contact holes 14a in the inter-layer insulator 11. The contact holes 14a are positioned overt the semiconductor layers 24. The resist pattern is removed. The contact holes 14a are formed to form contacts 14. Parts of the semiconductor layers 24 are exposed through the contact holes 14a. The semiconductor layers 24 are to be formed into the contact layers 12 and the upper diffusion layers 13. With reference to FIGS. 48A and 48B, a dopant is doped into the exposed portions of the semiconductor layers 24 through the contact holes 14a, thereby forming the upper diffusion layers 13 in the semiconductor layers 24 under the contact holes 14a. Further, a dopant is doped into the exposed portions of the semiconductor layers 24 through the contact holes 14a, thereby forming the contact layers 12 in the semiconductor layers 24 under the contact holes 14a. With reference again to FIGS. 49A and 49B, the contacts 14 are formed in the contact holes 14a, thereby completing the semiconductor device. When the semiconductor device shown in FIGS. 49A and 49B is formed by the processes shown in FIGS. 46A, 46B, 47A, 47B, 48A and 48B, the doping process to form the contact layers 12 or the contact implantation can not prevent that the dopant penetrates the upper diffusion layers 13 and reaches the pillars 2. Once the dopant for forming the contact layers 12 is doped into the pillars 2, a punch through phenomenon may be caused in the semiconductor device. Reducing the dose or the amount of the dopant into the semiconductor layers 24 may be effective to avoid the punch through problem that the dopant for forming the contact layers 12 is doped into the pillars 2. Reducing the dose or the amount of the dopant into the semiconductor layers 24 may cause another problem with the resistivity of the contact layers 12 between the upper diffusion layers 13 and the contacts 14. Namely, reducing the dose or the amount of the dopant into the semiconductor layers 24 may increase the contact resistance between the upper diffusion layers 13 and the contacts 14. Increasing the contact resistance between the upper diffusion layers 13 and the contacts 14 reduces the on-current of the semiconductor device. When the semiconductor device shown in FIGS. 49A and 49B is formed by the processes shown in FIGS. 46A, 46B, 47A, 47B, 48A and 48B, the doping process to form the upper diffusion layers 13 or the source/drain implantation can not prevent that the dopant reaches the pillars 2. Once the dopant for forming the upper diffusion layers 13 is doped into the pillars 2, the punch through phenomenon may be caused in the semiconductor device. Reducing the doping energy or the implantation energy in doping the dopant into the semiconductor layers 24 may be effective to avoid the punch through problem that the dopant for forming the upper diffusion layers 13 is doped into the pillars 2. Reducing the doping energy or the implantation energy in doping the dopant into the semiconductor layers 24 may cause another problem that the upper diffusion layers 13 are off-set from the gate electrodes 7. There is formed an off-set region between the upper diffusion layers 13 and the gate electrodes 7. When the vertical transistor turns ON, the off-set region is highly resistive. The highly resistive off-set region between the upper diffusion layers 13 and the gate electrodes 7 reduces the on-current of the semiconductor device. It had been desired to provide a semiconductor device which can avoid punch-through problem caused by the contact implantation and/or the source/drain implantation. The semiconductor device has high ON current characteristics. It had been desired to provide a method of forming such a semiconductor device. The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teaching of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purpose. Part First Embodiment: FIG. 13A is a fragmentary cross sectional elevation view, taken along a first direction, illustrating a semiconductor device in a first embodiment in accordance with the present invention. FIG. 13B is a fragmentary cross sectional elevation view, taken along a second direction perpendicular to the first direction, illustrating the semiconductor device of FIG. 13A. A semiconductor device shown in FIGS. 13A and 13B includes a semiconductor substrate 1 and a vertical transistor T over the semiconductor substrate 1. The vertical transistor T may include, but is not limited to, lower diffusion layers 6, pillars 2, upper diffusion layers 23, gate insulating films 8, and gate electrodes 7. The lower diffusion layers 6 are disposed in the semiconductor substrate 1. The pillars 2 are disposed over the lower diffusion layers 6. The upper diffusion layers 23 are disposed over the pillars 2. The gate insulating film 8 surrounds the pillar 2. The gate electrode 7 surrounds the gate insulating film 8 that surrounds the pillar 2. The gate electrode 7 is separated by the gate insulating film 8 from the pillar 2. The lower diffusion layers 6 may be realized by a doped silicon layer. The lower diffusion layers 6 can be formed by source/drain implantation process. The lower diffusion layers 6 are electrically separated by oxide films 5 and 5b from the gate electrodes 7. The pillars 2 provide channels. The pillars 2 may be realized by a doped silicon layer. The pillars 2 can be formed by channel implantation process. The upper diffusion layers 23 may be realized by a doped silicon layer. The upper diffusion layers 23 can be formed by source/drain implantation process. The upper diffusion layer 23 is surrounded by a side wall 9. The upper diffusion layer 23 is electrically separated by an oxide film 9a from the gate electrodes 7. FIG. 14 is a fragmentary plan view illustrating the semiconductor device of FIGS. 13A and 13B with an array of upper diffusion layers. FIG. 15 is a fragmentary plan view illustrating the semiconductor device of FIGS. 13A and 13B with an array of upper diffusion layers and conductive pads. FIG. 16 is a fragmentary plan view illustrating the semiconductor device of FIGS. 13A and 13B with an array of upper diffusion layers, conductive pads, and contacts. In FIGS. 14, 15 and 16, the line A-A′ is the first direction, along which the fragmentary cross sectional elevation view of FIG. 13A is taken, and the line B-B′ is the second direction, along which the fragmentary cross sectional elevation view of FIG. 13B is taken. The pillars 2 may each have a generally elliptical shape in plan view as shown in FIGS. 14, 15, and 16. The generally elliptical shape of each of the pillars 2 may have a major axis along the A-A′ line of FIGS. 14, 15, and 16, and a minor axis along the B-B′ line of FIGS. 14, 15, and 16. As shown in FIGS. 13A, 13B, 15 and 16, the semiconductor device may include conductive pads 15a. The conductive pads 15a are disposed over the upper diffusion layers 23. The upper diffusion layers 23 are upper portions of the vertical transistor T. The conductive pads 15a can be realized by an electrically conductive material. The conductive pads 15a may be formed of, but are not limited to, a doped polysilicon. The conductive pads 15a may each have, but is not limited to, a generally L-shape in plan view as shown in FIGS. 15 and 16. The conductive pads 15a may each include, but is not limited to, a transistor connection region 15b and a contact region 15c. The transistor connection region 15b overlaps the vertical transistor T in plan view. The contact region 15c does not overlap the vertical transistor T in plan view as shown in FIGS. 13A, 13B and 15. As shown in FIGS. 13A, 13B and 16, the semiconductor device may include contacts 10 which are electrically connected to the conductive pads 15a. The contacts 10 can be made of a conductive material. As shown in FIG. 16, the contacts 10 may each have a generally circular shape in plan view. As shown in FIG. 16, the contacts 10 may each be disposed over the contact region 15c and a part of the transistor connection region 15b of the conductive pads 15a. It is also possible that the contacts 10 may each overlap only the contact region 15c but does not overlap the transistor connection region 15b of the conductive pads 15a. The contacts 10 may each overlap at least a part of the contact region 15c of the conductive pads 15a. This arrangement of the contact 10 overlapping at least a part of the contact region 15c of the conductive pads 15a can be effective to reduce or prevent the punch-through effects which would be caused by the contact implantation and/or the source/drain implantation. The contacts 10 may each overlap entirely the contact region 15c of the conductive pads 15a. This arrangement of the contact 10 overlapping entirely the contact region 15c of the conductive pads 15a can be more effective to reduce or prevent the punch-through effects which would be caused by the contact implantation and/or the source/drain implantation. The contact 10 overlapping at least a part of the contact region 15c of the conductive pads 15a can be formed by making a contact hole 10a which is positioned over the at least part of the contact region 15c of the conductive pads 15a so that the at least part of the contact region 15c is exposed through the contact hole 10a. The contact 10 overlapping entirely the contact region 15c of the conductive pads 15a can be formed by making a contact hole 10a which is positioned over entirely the contact region 15c of the conductive pads 15a so that the contact region 15c is entirely exposed through the contact hole 10a. As shown in FIGS. 13A, 13B, 14, 15 and 16, the semiconductor device may have a two-dimensional array of sets of the vertical transistor T, the conductive pad 15a and the contact 10. The two-dimensional array may be a lattice with constant pitches, such as a matrix array. Each set of the vertical transistor T, the conductive pad 15a and the contact 10 may have the same arrangements of the conductive pad 15a and the contact 10 with respect to the vertical transistor T as shown in FIG. 16. The semiconductor device as described with reference to FIGS. 13A, 13B, 14, and 16 can be formed by, but not limited to, the following processes. FIG. 1A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step involved in a method of forming the semiconductor device of FIG. 13A. FIG. 1B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 13A. A semiconductor substrate 1 is prepared. A thermal oxidation process is carried out to form a thermal oxide film 3 on the semiconductor substrate 1. The thickness of the thermal oxide film 3 may be, but is not limited to, 10 nm. A dopant implantation process or a channel implantation process is carried out to selectively introduce a dopant into the semiconductor substrate 1 in a depth range for channel formation, thereby forming a channel implantation silicon region 2a. For example, the channel implantation process can be realized by, but is not limited to, the followings. Boron is implanted at implantation energy of 20 KeV and at a dose of 1E13/cm2. While the channel implantation silicon region 2a is formed in the semiconductor substrate 1, a non-doped silicon region 1a is formed in the semiconductor substrate 1, wherein the non-doped silicon region 1a is over the channel implantation silicon region 2a and under the thermal oxide film 3. A silicon nitride film 4 is formed over the thermal oxide film 3. The thickness of the silicon nitride film 4 may be, but is not limited to, 100 nm. The silicon nitride film 4 will be used as a hard mask in later process. FIG. 2A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 1A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 2B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 2A. A resist film is applied on the silicon nitride film 4. A lithography process is carried out to pattern the resist film, thereby forming a resist pattern over the silicon nitride film 4. A dry etching process is carried out using the resist pattern as a mask to selectively dry-etch the silicon nitride film 4, thereby forming a hard mask 4 of silicon nitride. The resist pattern is removed. A dry etching process is carried out using the hard mask 4 of silicon nitride to selectively dry-etch the thermal oxide film 3, so that the non-doped silicon region 1a is partially exposed. Dry etching processes are carried out using the hard mask 4 of silicon nitride to selectively dry-etch the non-doped silicon region 1a, the channel implantation silicon region 2a and the semiconductor substrate 1, thereby forming pillars 2 and grooves 5a. The pillars 2 are each made of the channel implantation silicon region 2a. The grooves 5a define the pillars 2 made of the channel implantation silicon region 2a. The grooves 5a extend to the deeper region than the pillars 2. The bottoms of the grooves 5a are deeper than the bottoms of the pillars 2. The depth of the grooves 5a may be, but is not limited to, 150 nm. FIG. 3A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 2A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 3B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 3A. A thermal oxidation process is carried out to oxidize the bottoms and side walls of the grooves 5a and side walls of the pillars 2, thereby forming an oxide film 5b. The oxide film 5b extends along the bottom and side walls of the grooves 5a and the side walls of the pillars 2. The oxide film 5b extending along the side walls of the pillars 2 is not illustrated. A high density plasma method is used to deposit an oxide film 5 over the silicon nitride hard mask 4 and in the grooves 5a so that the grooves 5a is partially filled with the oxide film 5. The lower part of the grooves 5a is filled with the oxide film 5. The lower part of the grooves 5a is lower in level than the pillars 2. The thickness of the grooves 5a may be, but is not limited to, 30 nm. Another dopant implantation process or a source/drain implantation process is carried out to selectively introduce a dopant into the semiconductor substrate 1 in a deeper depth range for source/drain formation, thereby forming lower diffusion layers 6 in the semiconductor substrate 1. For example, the source/drain implantation process can be realized by, but is not limited to, the followings. Arsenic is implanted at implantation energy of 30 KeV and at a dose of 5E15/cm2. The oxide films that are not illustrated and extend along the side walls of the pillars 2 are then removed by a wet etching process, so that the side walls of the pillars 2 are exposed. A gate oxidation process is carried out to firm gate insulating films 8. The gate insulating film 8 extends continuously along the side walls of the pillar 2 and is adjacent to an upper portion of the oxide film 5b. The upper portion of the oxide film 5b is adjacent to the boundary between the pillar 2 and the lower diffusion layers 6. Namely, the gate insulating film 8 extends along the side walls of the pillar 2 and extends in the upper portion of the lower diffusion layers 6. The upper portion of the lower diffusion layers 6 is adjacent to the pillar 2. The gate oxidation process can be carried out by a steam oxidation at a temperature of 1000° C. The thickness of the gate insulating films 8 may be, but are not limited to, 8 nm. FIG. 4A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 3A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 4B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 4A. A deposition process is carried out to deposit a polysilicon film entirely, while carrying out an in-situ doping of phosphorous. The thickness of the polysilicon film may be, but is not limited to, 50 nm. The phosphorus-doped polysilicon is then etched back by a dry etching process, thereby forming gate electrodes 7. The top of the etched-back polysilicon films or the gate electrodes 7 are lower in level than the top of the silicon nitride film 4 and higher in level than the bottom of the silicon nitride film 4. FIG. 5A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 4A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 5B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 5A. An oxide film is formed entirely over the semiconductor substrate. A chemical mechanical polishing process for polishing the oxide film is carried out using the silicon nitride film 4 as a stopper to planarize the oxide film, thereby forming an inter-layer insulator 22. FIG. 6A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 5A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 6B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 6A. The silicon nitride film 4 is removed by a wet etching process to form grooves 9b. The side walls of the upper portions of the polysilicon gate electrodes 7 are exposed to the grooves 9b. The side walls of the upper portions of the polysilicon gate electrodes 7 are oxidized to form oxide films 9a on the side walls of the upper portions of the polysilicon gate electrodes 7. The gate electrodes 7 are separated by the oxide films 9a from the grooves 9b. A silicon nitride film is formed along the thermal oxide film 3, the oxide films 9a, and the inter-layer insulator 22. The thickness of the silicon nitride film may be, but is not limited to, 10 nm. The silicon nitride film is etched back, thereby forming silicon nitride side walls 9. The silicon nitride side walls 9 extend along the oxide films 9a. The silicon nitride side walls 9 extend along the side walls of the grooves 9b. The tops of the silicon nitride side walls 9 are lower in level than the top of the inter-layer insulator 22 and higher in level than the tops of the polysilicon gate electrodes 7. FIG. 7A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 6A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 7B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 7A. A dry etching process is carried out to remove the oxide films 3 so that the non-doped silicon regions 1a are exposed. A selective epitaxial growth is carried out to form selective epitaxial layers 24 on the exposed surfaces of the non-doped silicon regions 1a. The selective epitaxial layers 24 are then planarized. The selective epitaxial layers 24 can be made of a polysilicon film. The thickness of the selective epitaxial layers 24 may be, but is not limited to, 20 nm. FIG. 8A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 7A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 8B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 8A. A polysilicon film 15 is formed entirely over the semiconductor substrate 1, while carrying out an in-situ doping of phosphorous into the polysilicon film 15. The polysilicon film 15 can be deposited by a chemical vapor deposition process. The thickness of the polysilicon film 15 may be, but is not limited to, 30 nm. Still another dopant implantation process or a second source/drain implantation process is carried out to selectively introduce a dopant into the selective epitaxial layers 24 and the non-doped silicon regions 1a, thereby forming upper diffusion layers 23. The upper diffusion layers 23 are formed over the pillars 2. For example, the second source/drain implantation process can be realized by, but is not limited to, the followings. Arsenic is implanted at implantation energy of 20 KeV and at a dose of 1E15/cm2. The oxide films that are not illustrated and extend along the side walls of the pillars 2 are then removed by a wet etching process, so that the side walls of the pillars 2 are exposed. As a result, the vertical transistors T are formed, which each include the lower diffusion layer 6, the pillar 2, and the upper diffusion layer 23. FIG. 9A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 8A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 9B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 9A. A resist film is formed over the polysilicon film 15. A lithography process is carried out to pattern the resist film, thereby forming a resist pattern over the polysilicon film 15. A dry etching process is carried out using the resist pattern as a mask to selectively etch the polysilicon film 15, thereby forming conductive pads 15d. The conductive pads 15d may each include, but is not limited to, a transistor connection region 15e and a contact region 15f. The transistor connection region 15e overlaps the vertical transistor T in plan view. The contact region 15f does not overlap the vertical transistor T in plan view. The resist pattern is removed. FIG. 10A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 9A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 10B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 10A. An inter-layer insulator 11 is formed over the conductive pads 15d and the inter-layer insulator 22. The inter-layer insulator 11 can be formed by a sputtering method. A chemical mechanical polishing process is then carried out to planarize the inter-layer insulator 11. FIG. 11A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 10A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 11B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 11A. A resist film is formed over the inter-layer insulator 11. A lithography process is carried out to pattern the resist film, thereby forming a resist pattern over the inter-layer insulator 11. A dry etching process is carried out using the resist pattern as a mask to selectively etch the inter-layer insulator 11, thereby forming contact holes 10a in the inter-layer insulator 11. The contact holes 10a are positioned over parts of the conductive pads 15d. The contact holes 10a are each positioned over a part of the transistor connection region 15e and a part of the contact region 15f. The resist film is then removed. The inter-layer insulator 11 has the contact holes 10a which expose the parts of the conductive pads 15d. The contact holes 10a each expose a part of the transistor connection region 15e and a part of the contact region 15f. FIG. 12A is a fragmentary cross sectional elevation view, taken along the first direction, illustrating a semiconductor device of the first embodiment in a step subsequent to the step of FIG. 11A involved in the method of forming the semiconductor device of FIGS. 13A and 13B. FIG. 12B is a fragmentary cross sectional elevation view, taken along the second direction, illustrating the semiconductor device of FIG. 12A. The conductive pads 15d are reduced in its resistivity. Yet another dopant implantation process or a contact implantation process is carried out using the inter-layer insulator 11 with the contact holes 10a as a mask to selectively introduce a dopant into the conductive pads 15d, thereby forming doped regions 15g in the conductive pads 15d. Namely, the contact implantation process is carried out to introduce the dopant through the contact holes 10a into the exposed parts of the transistor connection region 15e and the exposed part of the contact region 15f, thereby forming the doped regions 15g in the transistor connection region 15e and the doped regions 15g in the contact region 15f. The contact implantation process can be carried out by using phosphorous as the dopant at implantation energy of 20 KeV and at a dose of 3E16/cm2. With reference again to FIGS. 13A and 13B, a heat treatment is carried out to diffuse the dopant from the doped regions 15g to the entire region of the conductive pads 15d, and activate the polysilicon of the conductive pads 15d, thereby reducing the resistance of the conductive pads 15d. Diffusion of the dopant to the entire region of the conductive pads 15d and activation of the polysilicon of the conductive pads 15d results in forming the conductive pads 15a. The conductive pads 15a are electrically connected to the upper diffusion layers 23. The conductive pad 15a each includes, but is not limited to, a transistor connection region 15b and a contact region 15c. The transistor connection region 15b overlaps the vertical transistor T in plan view. The contact region 15c does not overlap the vertical transistor T in plan view as shown in FIGS. 13A, 13B and 15. As shown in FIGS. 13A, 13B and 16, the contact holes 10a are filled with a conductive material, thereby forming contacts 10 in contact holes 10a. The contacts 10 are electrically connected to the conductive pads 15a. As shown in FIG. 16, the contacts 10 may each have a generally circular shape in plan view. As shown in FIG. 16, the contacts 10 may each be disposed over a part of the contact region 15c and a part of the transistor connection region 15b of the conductive pads 15a. This arrangement of the contact 10 overlapping entirely the contact region 15c of the conductive pads 15a can be more effective to reduce or prevent the punch-through effects which would be caused by the contact implantation and/or the source/drain implantation. As shown in FIGS. 13A, 13B and 16, the semiconductor device includes the conductive pad 15a and the contact 10. The conductive pad 15a includes, but is not limited to, the transistor connection region 15b and the contact region 15c. The transistor connection region 15b overlaps the vertical transistor T in plan view. The transistor connection region 15b is eclectically connected to the vertical transistor T. The contact region 15c does not overlap the vertical transistor T in plan view. The contact 10 is electrically connected to the conductive pads 15a. The contact 10 is disposed over the contact region 15c of the conductive pads 15a. The conductive pads 15a can be formed by selectively introducing a dopant using the inter-layer insulator 11 with the contact holes 10a as a mask and by diffusing the dopant. These processes for forming the conductive pads 15a can not prevent that the dopant penetrates the upper diffusion layers 13 and reaches the pillars 2, thereby reducing or preventing any punch-through problem. This allows the vertical transistor T to show high performance. The conductive pad 15a is disposed between the contact 10 and the upper diffusion layers 23 being positioned over the pillar 2. The conductive pad 15a can be formed by selectively introducing a dopant using the inter-layer insulator 11 with the contact holes 10a as a mask and by diffusing the dopant. The conductive pad 15a can allow that the contact 10 does not overlap in plan view the upper diffusion layers 23 of the vertical transistor T, through the contact 10 is electrically connected through the conductive pad 15a to the upper diffusion layers 23 of the vertical transistor T. This makes it possible to reduce the amount of a dopant to be introduced into the pillar 2 which provides the channel of the vertical transistor T. Differently from this embodiment, if no conductive pad 15a were provided between the contact 10 and the upper diffusion layers 23 being positioned over the pillar 2, then the dopant to would reach the pillars 2. Once the dopant would be doped into the pillar 2, a punch through phenomenon may be caused in the vertical transistor T. The conductive pad 15a can reduce the amount of a dopant to be introduced into the pillar 2 which provides the channel of the vertical transistor T, as compared to when no conductive pad 15a is provided, wherein the vertical transistor T is positioned directly under the contact 10. Differently from this embodiment, it is assumed that no conductive pad 15a was provided between the contact 10 and the upper diffusion layers 23 being positioned over the pillar 2. It is assumed that the vertical transistor T is positioned directly under the contact 10. In this case, the contact implantation process is carried out for introducing a dopant directly into the upper portion of the vertical transistor T. This may allow that the dopant to would reach the pillars 2. Once the dopant would be doped into the pillar 2, a punch through phenomenon may be caused in the vertical transistor T. In accordance with this embodiment, the conductive pad 15a is disposed between the contact 10 and the upper diffusion layer 23 being positioned over the pillar 2. The contact 10 is connected through the conductive pad 15a to the upper diffusion layer 23 being positioned over the pillar 2. The contact 10 is connected through the conductive pad 15a to the upper portion of the vertical transistor T. The contact 10 is positioned mainly and directly over the contact region 15c of the conductive pad 15a. The upper diffusion layer 23 being positioned over the pillar 2 is positioned mainly and directly under the transistor connection region 15b of the conductive pad 15a. The contact 10 is electrically connected through the conductive pad 15a to the upper diffusion layer 23 and the pillar 2 of the vertical transistor T. The conductive pad 15a allows that the contact 10 does not largely overlap the upper diffusion layer 23 and the pillar 2 of the vertical transistor T. The conductive pad 15a can reduce an overlapping area in plan view between the contact 10 and the upper diffusion layer 23 and the pillar 2 of the vertical transistor T. The conductive pad 15a can allow the contact 10 does not overlap the upper diffusion layer 23 and the pillar 2 of the vertical transistor T. Reduction of the overlapping area in plan view between the contact 10 and the upper diffusion layer 23 and the pillar 2 of the vertical transistor T results in reduction of the amount of a dopant to be introduced into the upper diffusion layer 23 and the pillar 2 of the vertical transistor T during the contact implantation process. No overlapping area in plan view between the contact 10 and the upper diffusion layer 23 and the pillar 2 of the vertical transistor T results in almost no amount of a dopant to be introduced into the upper diffusion layer 23 and the pillar 2 of the vertical transistor T during the contact implantation process. No overlapping area or such a reduced overlapping area in plan view between the contact 10 and the vertical transistor T results as follows. Almost no amount or a reduced amount of a dopant is introduced into the vertical transistor T during the contact implantation process, even if a sufficiently large amount of a dopant is introduced into the conductive pad 15a during the contact implantation process. The conductive pad 15a is sufficiently reduced in its contact resistance with the contact 10. Any punch through due to the contact implantation process can be avoided effectively. Thus, the vertical transistor T can show high performances. As shown in FIGS. 13A, 13B, 15 and 16, the semiconductor device includes conductive pad 15a which further includes the transistor connection region 15b and the contact region 15c. The transistor connection region 15b overlaps the vertical transistor T in plan view. The contact region 15c does not overlap the vertical transistor T in plan view. It is possible to adjust or modify the shape and the dimension of the conductive pads 15a, particularly, the shape and the dimension of the contact region 15c of the conductive pad 15a. Adjustment or modification to the shape and the dimension of the conductive pad 15a, particularly, the contact region 15c can adjust or modify the overlapping area in plan view between the contact 10 and the upper diffusion layer 23 and the pillar 2 of the vertical transistor T. The conductive pad 15a ensures the electrical connection between the contact 10 and the upper diffusion layer 23 and the pillar 2 of the vertical transistor T, while avoiding that a certain amount of a dopant is introduced into the upper diffusion layer 23 and the pillar 2 of the vertical transistor T during the contact implantation process. Any punch through due to the contact implantation process can be avoided effectively. Thus, the vertical transistor T can show high performances. The conductive pad 15a can keep a sufficiently high design flexibility of the semiconductor device. In accordance with this embodiment, the pad formation process is that the conductive pad 15d including the transistor connection region 15e and the contact region 15f is formed so that the transistor connection region 15e overlaps the vertical transistor T in plan view, while the contact region 15f does not overlap the vertical transistor T in plan view. The resistance reduction processes include the contact implantation process and the diffusion of the dopant. At lest a part of the transistor connection region 15e is covered by the inter-layer insulator 11 having the contact holes 10a. The contact implantation process is carried out using the inter-layer insulator 11 with the contact holes 10a as a mask to selectively introduce a dopant into the conductive pads 15d, thereby forming doped regions 15g in the conductive pads 15d. The heat treatment is carried out to diffuse the dopant from the doped regions 15g to the entire region of the conductive pads 15d, and activate the polysilicon of the conductive pads 15d, thereby reducing the resistance of the conductive pads 15d. The contact formation process is that the contacts 10 are formed in contact holes 10a. The contacts 10 are disposed over the contact region 15f of the conductive pad 15d. The contact implantation process is carried out to introduce a dopant through the contact hole 10a into the doped regions 15g in the conductive pads 15d. The contact hole 10a does not largely overlap or does not overlap the vertical transistor T. The conductive pads 15d including the transistor connection region 15e and the contact region 15f can reduce an overlapping area in plan view between the contact hole 10a and the upper diffusion layer 23 and the pillar 2 of the vertical transistor T. The conductive pad 15d can allow the contact hole 10a does not overlap the upper diffusion layer 23 and the pillar 2 of the vertical transistor T. Reduction of the overlapping area in plan view between the contact hole 10a and the upper diffusion layer 23 and the pillar 2 of the vertical transistor T results in reduction of the amount of a dopant to be introduced into the upper diffusion layer 23 and the pillar 2 of the vertical transistor T during the contact implantation process. No overlapping area in plan view between the contact hole 10a and the upper diffusion layer 23 and the pillar 2 of the vertical transistor T results in almost no amount of a dopant to be introduced into the upper diffusion layer 23 and the pillar 2 of the vertical transistor T during the contact implantation process. No overlapping area or such a reduced overlapping area in plan view between the contact hole 10a and the vertical transistor T results as follows. Almost no amount or a reduced amount of a dopant is introduced into the vertical transistor T during the contact implantation process, even if a sufficiently large amount of a dopant is introduced into the conductive pad 15a during the contact implantation process. The conductive pad 15a is sufficiently reduced in its contact resistance with the contact 10. Any punch through due to the contact implantation process can be avoided effectively. Thus, the vertical transistor T can show high performances. Part Second Embodiment: FIG. 23A is a fragmentary cross sectional elevation view, taken along a first direction, illustrating a semiconductor device in a second embodiment in accordance with the present invention. FIG. 23B is a fragmentary cross sectional elevation view, taken along a second direction perpendicular to the first direction, illustrating the semiconductor device of FIG. 23A. The semiconductor device shown in FIGS. 23A and 23B is different from the semiconductor device shown in FIGS. 13A and 13B only in that conductive pads 25a are formed by reducing resistance of selective epitaxial silicon layers. As shown in FIGS. 23A, 23B, 15 and 16, the semiconductor device may include conductive pads 25a. The conductive pads 25a are disposed over the upper diffusion layers 23. The upper diffusion layers 23 are upper portions of the vertical transistor T. The conductive pads 25a may be formed of, but are not limited to, an epitaxial silicon layer. The conductive pads 25a may each have, but is not limited to, a generally L-shape in plan view as shown in FIGS. 15 and 16. The conductive pads 25a may each include, but is not limited to, a transistor connection region 25b and a contact region 25c. The transistor connection region 25b overlaps the vertical transistor T in plan view. The contact region 25c does not overlap the vertical transistor T in plan view as shown in FIGS. 23A, 23B and 15. As shown in FIGS. 13A, 13B and 16, the semiconductor device may
{'bibliographic_information': "{'country': 'US', 'doc-number': '12621104', 'kind': 'B2', 'date': '20091118', 'extension': '94', 'invention_title': 'Semiconductor device with transistor, conductive pad, and contact'}", 'source_file': './grant/2012/ipg120207.xml', 'source_data': 'USPTO-Grant'}