Method of Making a Thin Film Transistor

This invention relates to thin film transistors, and more particularly to a structure and fabricating method of a thin film transistor suitable for devices such as a SRAM memory cell.

In general, P-type thin film transistors are used in SRAM devices of over 1M bit density in place of a load resistor. N-type thin film transistors are widely used in liquid crystal displays as a switching element for switching video data signals of each pixel region. To fabricate a high quality SRAM, a thin film transistor has to have its off current reduced and its on current increased in order for the SRAM to have reduced power consumption and improved memory characteristics. Based on the foregoing principles, studies on improving the on/off current ratio are actively underway.

A conventional method for fabricating a thin film transistor for improving the on/off current ratio based on the foregoing principles will be explained hereinafter, referring to the attached drawings. FIGS. 1A-1F are sections illustrating conventional fabricating processes of a thin film transistor.

A conventional fabricating process of a thin film transistor is as follows. As illustrated in FIG. 1A, gate electrode 2 is formed on insulation substrate 1 and gate insulation film 3 is formed on the surface thereof. Then, as illustrated in FIG. 1B, intrinsic semiconductor layer 4 is deposited on the overall surface thereof. As illustrated in FIG. 1C, channel ions (N-type ions) are injected into intrinsic semiconductor layer 4. As illustrated in FIG. 1D, after depositing first oxide film 5 on the overall surface thereof and carrying out a patterning (with a photoetching process) of first oxide film 5 to leave first oxide film 5 only on the channel region over gate electrode 2, low density source/drain ions (P-) are injected into intrinsic semiconductor layer 4 with patterned first oxide film 5 used as a mask.

As illustrated in FIG. 1E, after forming second oxide film side walls 7 through depositing a thick second oxide film on the overall surface thereof and carrying out an anisotropic etching of the second oxide film, high density source/drain ions (P+) are injected into intrinsic semiconductor layer 4 with patterned first oxide film 5 and second oxide film side walls 7 used as masks. As illustrated in FIG. IF, by removing first oxide film 5 and second oxide film side walls 7, a thin film transistor of an LDD (Lightly Doped Drain) structure is completed.

The foregoing conventional thin film transistor, however, has the following problems. The thin film transistor has problems of low yield and degradation of characteristics due to the complicated processes and the increased number of steps resulting from the additionally required mask fabrication processes as well as the mask removal processes for forming the LDD structures of the source/drain regions.

The object of this invention, which is devised for solving the foregoing problems, may improve the yield and the characteristics of a thin film transistor by reducing the required number of processes.

These and other objects and features of this invention can be achieved by providing a thin film transistor structure including: an insulation substrate; a gate electrode formed on the insulation substrate; a gate insulation film formed on the gate electrode and on the insulation substrate; a semiconductor layer formed on the gate insulation film; channel regions formed in portions of the semiconductor layer at the sides of the gate electrode; a high density first conductive type first impurity region formed in the semiconductor layer over the gate electrode; and first conductive type second impurity regions having an LDD structure formed in portions of the semiconductor layer over the insulation substrate except under the gate electrode.

Objects of the present invention also are achieved by providing a method for fabricating a thin film transistor including processes for: forming a gate electrode on an insulation substrate and forming a gate insulation film on the overall surface thereof; forming a semiconductor layer on the gate insulation film; vertically injecting low density source/drain impurity ions into the semiconductor layer; forming insulation film side walls at the side walls of the semiconductor layer at the sides of the gate electrode; and injecting high density source/drain ions into the semiconductor layer with the insulation film side walls used as masks, such as described in parent application application Ser. No. 08/356,822, which is hereby incorporated by reference.

Other objects of the present invention are achieved by providing a method for fabricating a thin film transistor including processes for: forming a gate electrode having first and second sides on an insulation substrate; forming a gate insulation film on the gate electrode and the substrate; forming a semiconductor layer on the gate insulation film; forming diffusion-preventing spacers on the semiconductor layer corresponding or adjacent to the first and second sides of the gate electrode; forming impurity-containing spacers on the diffusion-preventing spacers and the semiconductor layer adjacent thereto; forming a low density impurity region in the semiconductor layer by diffusing impurities from the impurity-containing spacers; and injecting impurities in the semiconductor layer using the impurity-containing spacers as a mask to form high density impurity regions.

FIGS. 1A-1F are sections illustrating a conventional process for fabricating a thin film transistor.

FIG. 2 is a perspective view of a thin film transistor generally in accordance with this invention.

FIGS. 3A-3E are sections illustrating a process for fabricating a thin film transistor in accordance with this invention.

Preferred embodiments of the present invention will be explained hereinafter in detail, referring to the attached drawings.

FIG. 2 is a perspective view of a thin film transistor generally in accordance with this invention. FIGS. 3A-3E are sections illustrating a process for fabricating a thin film transistor in accordance with this invention.

As illustrated in FIG. 2, a thin film transistor structure in accordance with this invention includes: gate electrode 2 formed on insulation substrate 1 (which may be a substrate formed of an insulative material or a conductive or semiconductive substrate on which is formed an insulator); gate insulation film 3, which may be an oxide film, formed on gate electrode 2 and on insulation substrate 1; semiconductor layer 4 formed on gate insulation film 3; channel regions 11 of a transistor formed in parts of semiconductor layer 4 at both sides of gate electrode 2; insulation film side walls 7 formed on semiconductor layer 4 along the sides of gate electrode 2; high density P-type first impurity region 9 formed in the semiconductor layer over gate electrode 2, and P-type second impurity regions 10a and 10b having an LDD structure formed in parts of semiconductor layer 4 over the substrate except under gate electrode 2.

A method for fabricating a thin film transistor in accordance with this invention having a structure similar to that discussed above will be explained hereinafter.

As illustrated in FIG. 3A, gate electrode 2 is formed on substrate 1 and gate oxide film 3 is formed on gate electrode 2 and substrate 1. Gate electrode 2 preferably is formed of an N-type doped polysilicon. Since the thickness (height) of gate electrode 2 serves to define the length of the channel region, the thickness of gate electrode 2 is adjusted considering the desired channel length.

As illustrated in FIGS. 3B and 3C, semiconductor layer 4 is formed on gate oxide film 3, and oxide film 12 (which may be about 100-500 .ANG. in thickness) is formed on semiconductor layer 4. Oxide film 12 may be formed by an oxidation process to form a thermal oxidation film on the surface of the semiconductor layer or by deposition of a CVD oxide. As illustrated in FIG. 3D, oxide film 12 is subjected to anistropic etching to form oxide film sidewalls 13 on the surface of semiconductor layer 4 opposite to the sides of gate electrode 2, and BSG (Boron Silicate Glass) layer 14 is formed on oxide film sidewalls 13 and semiconductor layer 4.

Referring to FIG. 3E, oxide film sidewalls 13 are used as diffusion-preventing layers for preventing the boron from being diffused to a channel region during diffusion of the boron in the BSG film into portions of semiconductor layer 4 that are in contact with BSG layer 14. BSG layer 14 is formed to a thickness of about 2000 .ANG. or over, with a concentration of the boron therein being in the range of about 1.times.10.sup.19 -1.times.10.sup.21 atoms/cm.sup.3. BSG layer 14 is subjected to anistropic etching to form BSG sidewalls 15 on oxide film sidewalls 13 and on semiconductor layer 4 adjoining oxide film sidewalls 13, and subjected to annealing at an elevated temperature over about 800.degree. C. for more than about 10 minutes in a nitrogen ambient to diffuse the boron impurities in BSG sidewalls 15 into portions of semiconductor layer 4 that are in contact with BSG sidewalls 15, to form lightly doped P-type impurity regions in semiconductor layer 4. Ions are injected into semiconductor layer 4 using BSG sidewalls 15 as masks to form heavily doped P-type impurity regions in semiconductor layer 4 as illustrated. The width of each of BSG sidewalls 15 is greater than about 2000 .ANG., the concentration of the resulting lightly doped P-type impurity regions is about 1.times.10.sup.18 -1.times.10.sup.20 atoms/cm.sup.3, and the dose in the formation of the heavily doped P-type impurity regions is about 1.times.10.sup.13 -1.times.10.sup.16 atoms/cm.sup.2, which is equivalent to a concentration of about 1.times.10.sup.20 -1.times.10.sup.22 atoms/cm.sup.3. BSG may be applied for forming the lightly doped P-type impurity regions in the case of a PMOS device, and PSG (Phosphorous Silicate Glass) may be applied for forming the lightly doped N-type impurity regions in the case of an NMOS device.

A preferred embodiment of the present invention uses BSG sidewalls in the formation of the lightly doped P-type impurity regions, which may serve to prevent an adverse effect on the channel region, which is formed vertically. If high energy ions are injected, for example, the impurities may be diffused even to the channel region, resulting in an undesired change of the Vt (Threshold Voltage), which may degrade the performance of the device. The gate electrode may be doped with P-type (boron) impurities in case of a PMOS device, and with N-type (phosphorous) impurities in case of an NMOS device for obtaining a stable Vt characteristic.

Moreover, if an impurity region is formed with such a process, a drain region, being offset to the gate electrode, is produced with an LDD structure.

The structure and fabricating method of a thin film transistor in accordance with this invention as in the foregoing explanation has the following advantages.

First, since it is possible to form a drain region of an LDD structure in self-alignment without a masking process, a thin film transistor having constant characteristics can be obtained irrespective of variation of the process.

Second, since no masking process as well as no removal process are required, the process is simple and the yield is increased.

Third, since the channel is vertically formed, even though the design rule may be scaled down, a thin film transistor having a fixed length channel can be secured.

Another embodiment for forming a thin film transistor of such a structure in accordance with the present invention is disclosed in FIGS. 3A to 3D of parent application Ser. No. 08/356,822, which is hereby incorporated by reference.

Although the invention has been described in conjunction with specific embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all of the alternatives and variations that fall within the spirit and scope of the appended claims.