Method of forming a transistor structure

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for fabricating a semiconductor device and, more particularly, to a semiconductor device in which polysilicon is used to form source and drain regions in an initial process step so as to reduce resistance of bit lines and minimize a junction capacitance and thus improve its reliability, and a method for fabricating the same.

[0004] 2. Discussion of the Related Art Due to buried N⁺ layers(hereinafter referred to as BN⁺ layer), which is used in flash EEPROM cells and mask ROM cells, it is possible to make virtual ground, and the number of contact holes is diminished to reduce sizes of chips. Source and drain regions are commonly formed in a final process step in a case a semiconductor device is fabricated using BN⁺ layers.

[0005] A conventional method for fabricating a semiconductor device will be described with reference to FIGS. 1A to 1C which are cross-sectional views showing process steps of a method for fabricating a semiconductor device.

[0006] First, a photoresist film is coated on a semiconductor substrate 11 or a p well as shown in FIG. 1A. In this case, instead of a photoresist film 13 alone being coated, an insulating layer is first deposited on a semicondutor substrate 11 and then a photoresist film 13 is coated on the insulating layer.

[0007] Next, as shown in FIG. 1B, the photoresist film 13 over placements of BN⁺ layers is patterned by exposure and development processes. N⁺ heavily doped impurity ions are implanted into the semiconductor substrate 11 by using the photoresist pattern 13 as a mask.

[0008] Subsequently, the remaining photoresist film 13 is removed as shown in FIG. 1C. Then an annealing process is performed to grow a gate oxide layer 15 on the semiconductor substrate 11 and simultaneously form BN⁺ layers 17. At this time, the BN⁺ layers 17 function as source and drain regions. The gate oxide layer 15 is thicker on the BN⁺ layers than on the semiconductor substrate 11 because the BN⁺ layers have a high concentration of impurity ions.

[0009] According to a conventional method for fabricating a semiconductor device, source and drain regions are formed in an initial process step and gate electrodes are formed right over the source and drain regions so that it can be possible to make virtual ground and the number of contact holes in a cell is minimized to reduce the area of a chip.

[0010] However, it has following problems. There is a reduced a voltage applied to bitlines in contact with BN⁺ layers due to a resistance of the BN⁺ layers. Moreover, since a junction area of the BN⁺ layers and a semiconductor substrate increases and thus a junction capacitance on the junction area increases, operational speed of the semiconductor device is lowered.

[0011] Therefore, the present invention is directed to a structure of a semiconductor device and a method for fabricating the same that substantially obviates one or more of problems due to limitations and disadvantages of the related art.

[0012] An object of the invention is to provide a method for fabricating a semiconductor device in which BN⁺ layers are formed of doped polysilicon to be used as source and drain regions so as to reduce a resistance of bitlines and minimize a junction capacitance and thus increase its reliability.

[0013] Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0014] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a semiconductor device includes a semiconductor substrate, trenches formed in predetermined areas of the semiconductor substrate, an insualting layer formed beneath a surface of each of the trenches to have a recess, a polysilicon layer formed on the insualting layer in the trench, source and drain regions formed at both sides of the polysilicon layer beneath a surface of the semiconductor substrate, and gates formed over the semiconductor substrate.

[0015] In another aspect of the present invention, a method for fabricating a semiconductor device includes the steps of forming trenches in predetermined areas of a semiconductor substrate, forming a first polysilicon layer doped by impurity ions in each of the trenches, diffusing the impurity ions in the first polysilicon layer so as to form source and drain regions at both sides of each of the trenches, and forming a second polysilicon layer on the entire surface and then patterning the second polysilicon layer to form gates.

[0016] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

[0017] These and various other objects, features, and advantages of the present invention will be readily understood with reference to the following detailed description read in conjunction with the accompanying drawings, in which:

[0018] FIGS. 1A to 1C are cross-sectional views showing process steps of a conventional method for fabricating a semiconductor device;

[0019] FIG. 2 is a cross-sectional view showing a structure of a semiconductor device according to the present invention; and

[0020] FIGS. 3A to 3J are cross-sectional views showing process steps of a method for fabricating a semiconductor device according to a preferred embodiment of the present invention.

[0021] According to the present invention, BN⁺ layers are formed of polysilicon to reduce a resistance of bitlines, and junction areas of the BN⁺ layers and the semiconductor substrate is minimized, thereby reducing a junction capacitance. Such a semiconductor device of the present invention can be widely applied to flash memories, mask ROMs, and EEPROMs, all of which require high speed performance.

[0022] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[0023] FIG. 2 is a cross-sectional view showing a structure of a semiconductor device according to the present invention. As shown in FIG. 2, a semiconductor device of the present invention includes a semiconductor substrate, trenches formed in predetermined areas of the semiconductor substrate, an insualting layer formed beneath a surface of each of the trenches to have a recess, a polysilicon layer formed on the insulating layer in each of the trenches, source and drain regions formed beneath the surface of the semiconductor substrate at both sides of the polysilicon layer, and gates formed over the semiconductor substrate. In this case, an insualting layer is placed between the gates and the polysilicon layers. That is, since the polysilicon layer is doped by impurity ions, the polysilicon layer is activated more greatly than the other layer so that the insulating layer is formed to be thicker on the polysilicon layer than on the other area.

[0024] A method for fabricating a semiconductor device having the aforementioned structure will be described with reference to FIGS. 3A to 3J which are cross-sectional views showing process steps of a method for fabricating a semiconductor device. This embodiment is manifested on a basis of a flash EEPROM.

[0025] Referring initially to FIG. 3A, a pad oxide layer 19 is grown on a semiconductor substrate 11 and then a first insulating layer 21 is formed on the entire surface. This insulating layer 21 is formed of any one of a silicon nitride layer, an oxide layer, and a photoresist film. Then a photoresist film is coated on the first insulating layer 21 and then is patterned by exposure and development processes to expose the semiconductor substrate 11 over placements of BN⁺ layers. Subsequently, using the photoresist pattern as a mask, the first insualting layer 21, the pad oxide layer 19, and the semiconductor substrate 11 are etched to a predetermined depth, thereby forming trenches.

[0026] Referring to FIG. 3B, the remaining photoresist film is removed and then a second insulating layer 25 is deposited on the entire surface. At this time, this second insualting layer 25 is deposited by a chemical vapor deposition (CVD) process so as to reduce a junction capacitance by minimizing a junction area of the semiconductor substrate and a polysilicon layer which will be formed in a following process step.

[0027] Referring to FIG. 3C, the second insulating layer 25 is etched-back by a predetermined depth to remain in the trenches. At this time, the second insulating layer 25 doesn't remain be as high as the surface of the semiconductor substrate 11. Thus, polysilicon will fill the rest of the trench as high as the surface of the substrate 11.

[0028] Referring to FIG. 3D, a third insulating layer is deposited on the entire surface including the second insulating layer 25 and then is etched-back to form a sidewall spacer 27 on the side surface of each of the trenches. Thereafter, using the sidewall spacers 27 as masks, the second insulating layer 25 in each of the trenches is etched by a predetermined depth to have a recess. At this time, the recess is for minimizing a junction area of the semiconductor substrate 11 and a polysilicon layer and maximizing an amount of impurity ions which will be diffused from the polysilicon layer.

[0029] Referring to FIG. 3E, the sidewall spacers 27 are removed, and; a first polysilicon layer 29 is formed on the entire surface including the second insulating layer 25. This first polysilicon layer 29 used is a polysilicon layer that is heavily doped by impurity ions.

[0030] Referring to FIG. 3F, the first polysilicon layer 29 and the pad oxide layer 19 above the surface of the semiconductor substrate 11 are removed so that the first polysilicon layer 29 and the second insulating layer 25 are buried in the trenches. The thickness of the first polysilicon layer 29 is determined taking into account the etch ratio of the first polysilicon layer 29 and the pad oxide layer 19 such that there is no problem with etching the polysilicon layer 29 only above the semiconductor substrate 11.

[0031] Referring to FIG. 3G, a gate insulating layer 31 is grown on the entire surface of the semiconductor substrate 11. The gate insulating layer 31 has a thicker thickness on the first polysilicon layer 29 than on the semiconductor substrate 11 because the impurity ions in the first polysilicon layer 29 are actively diffused by an annealing process. By this annealing process, source S and drain D regions are formed at both sides of the first polysilicon layer 29.

[0032] Referring to FIG. 3H, a second polysilicon layer is deposited on the gate insulating layer 31 and then selectively removed to form a plurality of floating gates 33 over predetermined areas of the semiconductor substrate 11. Next, a fourth insulating layer 35 is deposited on the entire surface of the floating gates 33 and then a third polysilicon layer 37 used as a control gate is formed on the fourth insulating layer 35. This fourth insulating layer 35 functions as an interpoly insulating layer that electrically insulates the floating gates 33 from the third polysilicon layer 37.

[0033] Referring to FIG. 3I, a fifth insulating layer 39, which is a high temperature low pressure dielectric (HLD) layer, is formed on the third polysilicon layer 37 used as a control gate.

[0034] Referring finally to FIG. 3J, a fourth polysilicon layer 41 used as an erasable gate is deposited on the fifth insulating layer 39, thereby completing the process of a semiconductor device according to the present invention.

[0035] The method for fabricating a semiconductor device has advantages. First since polysilicon layers are used as BN⁺ layers in contact with bitlines, a resistance of the bitlines is reduced and thus a voltage applied to the bitlines is reduced. Further, since the BN⁺ layers are formed by doped polysilicon, not by diffusion of impurity ions, a junction area of a semiconductor substrate and the polysilicon is reduced. And a junction capacitance is minimized to improve the operational speed of the semiconductor device.

[0036] It will be apparent to those skilled in the art that various modification and variations can be made in the semiconductor device and a method for fabricating the same of the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.